diff --git a/configure.ac b/configure.ac
index ebfd52f62a..fa62b5fa85 100644
--- a/configure.ac
+++ b/configure.ac
@@ -9,43 +9,43 @@ tooldir="$srcdir/tool"
 
 AC_DISABLE_OPTION_CHECKING
 
-m4_include([tool/m4/_colorize_result_prepare.m4])
-m4_include([tool/m4/ac_msg_result.m4])
-m4_include([tool/m4/colorize_result.m4])
-m4_include([tool/m4/ruby_append_option.m4])
-m4_include([tool/m4/ruby_append_options.m4])
-m4_include([tool/m4/ruby_check_builtin_func.m4])
-m4_include([tool/m4/ruby_check_builtin_setjmp.m4])
-m4_include([tool/m4/ruby_check_printf_prefix.m4])
-m4_include([tool/m4/ruby_check_setjmp.m4])
-m4_include([tool/m4/ruby_check_signedness.m4])
-m4_include([tool/m4/ruby_check_sizeof.m4])
-m4_include([tool/m4/ruby_check_sysconf.m4])
-m4_include([tool/m4/ruby_cppoutfile.m4])
-m4_include([tool/m4/ruby_decl_attribute.m4])
-m4_include([tool/m4/ruby_default_arch.m4])
-m4_include([tool/m4/ruby_define_if.m4])
-m4_include([tool/m4/ruby_defint.m4])
-m4_include([tool/m4/ruby_dtrace_available.m4])
-m4_include([tool/m4/ruby_dtrace_postprocess.m4])
-m4_include([tool/m4/ruby_func_attribute.m4])
-m4_include([tool/m4/ruby_mingw32.m4])
-m4_include([tool/m4/ruby_prepend_option.m4])
-m4_include([tool/m4/ruby_prog_gnu_ld.m4])
-m4_include([tool/m4/ruby_replace_funcs.m4])
-m4_include([tool/m4/ruby_replace_type.m4])
-m4_include([tool/m4/ruby_rm_recursive.m4])
-m4_include([tool/m4/ruby_setjmp_type.m4])
-m4_include([tool/m4/ruby_stack_grow_direction.m4])
-m4_include([tool/m4/ruby_try_cflags.m4])
-m4_include([tool/m4/ruby_try_cxxflags.m4])
-m4_include([tool/m4/ruby_try_ldflags.m4])
-m4_include([tool/m4/ruby_universal_arch.m4])
-m4_include([tool/m4/ruby_werror_flag.m4])
+m4_include([tool/m4/_colorize_result_prepare.m4])dnl
+m4_include([tool/m4/ac_msg_result.m4])dnl
+m4_include([tool/m4/colorize_result.m4])dnl
+m4_include([tool/m4/ruby_append_option.m4])dnl
+m4_include([tool/m4/ruby_append_options.m4])dnl
+m4_include([tool/m4/ruby_check_builtin_func.m4])dnl
+m4_include([tool/m4/ruby_check_builtin_setjmp.m4])dnl
+m4_include([tool/m4/ruby_check_printf_prefix.m4])dnl
+m4_include([tool/m4/ruby_check_setjmp.m4])dnl
+m4_include([tool/m4/ruby_check_signedness.m4])dnl
+m4_include([tool/m4/ruby_check_sizeof.m4])dnl
+m4_include([tool/m4/ruby_check_sysconf.m4])dnl
+m4_include([tool/m4/ruby_cppoutfile.m4])dnl
+m4_include([tool/m4/ruby_decl_attribute.m4])dnl
+m4_include([tool/m4/ruby_default_arch.m4])dnl
+m4_include([tool/m4/ruby_define_if.m4])dnl
+m4_include([tool/m4/ruby_defint.m4])dnl
+m4_include([tool/m4/ruby_dtrace_available.m4])dnl
+m4_include([tool/m4/ruby_dtrace_postprocess.m4])dnl
+m4_include([tool/m4/ruby_func_attribute.m4])dnl
+m4_include([tool/m4/ruby_mingw32.m4])dnl
+m4_include([tool/m4/ruby_prepend_option.m4])dnl
+m4_include([tool/m4/ruby_prog_gnu_ld.m4])dnl
+m4_include([tool/m4/ruby_replace_funcs.m4])dnl
+m4_include([tool/m4/ruby_replace_type.m4])dnl
+m4_include([tool/m4/ruby_rm_recursive.m4])dnl
+m4_include([tool/m4/ruby_setjmp_type.m4])dnl
+m4_include([tool/m4/ruby_stack_grow_direction.m4])dnl
+m4_include([tool/m4/ruby_try_cflags.m4])dnl
+m4_include([tool/m4/ruby_try_cxxflags.m4])dnl
+m4_include([tool/m4/ruby_try_ldflags.m4])dnl
+m4_include([tool/m4/ruby_universal_arch.m4])dnl
+m4_include([tool/m4/ruby_werror_flag.m4])dnl
 
-AC_ARG_VAR([cflags], [additional CFLAGS (ignored when CFLAGS is given)])
-AC_ARG_VAR([cppflags], [additional CPPFLAGS (ignored when CPPFLAGS is given)])
-AC_ARG_VAR([cxxflags], [additional CXXFLAGS (ignored when CXXFLAGS is given)])
+AC_ARG_VAR([cflags], [additional CFLAGS (ignored when CFLAGS is given)])dnl
+AC_ARG_VAR([cppflags], [additional CPPFLAGS (ignored when CPPFLAGS is given)])dnl
+AC_ARG_VAR([cxxflags], [additional CXXFLAGS (ignored when CXXFLAGS is given)])dnl
 
 : "environment section" && {
 HAVE_BASERUBY=yes
diff --git a/tool/m4/_colorize_result_prepare.m4 b/tool/m4/_colorize_result_prepare.m4
index cc2bbaa703..c8478bd0b6 100644
--- a/tool/m4/_colorize_result_prepare.m4
+++ b/tool/m4/_colorize_result_prepare.m4
@@ -1,4 +1,4 @@
-# -*- Autoconf -*-
+dnl -*- Autoconf -*-
 AC_DEFUN([_COLORIZE_RESULT_PREPARE], [
     msg_checking= msg_result_yes= msg_result_no= msg_result_other= msg_reset=
     AS_CASE(["x${CONFIGURE_TTY}"],
diff --git a/tool/m4/ac_msg_result.m4 b/tool/m4/ac_msg_result.m4
index bcc7a63d7d..12a3617c0d 100644
--- a/tool/m4/ac_msg_result.m4
+++ b/tool/m4/ac_msg_result.m4
@@ -1,4 +1,4 @@
-# -*- Autoconf -*-
+dnl -*- Autoconf -*-
 AC_DEFUN([AC_MSG_RESULT], [dnl
 { _AS_ECHO_LOG([result: $1])
 COLORIZE_RESULT([$1]); dnl
diff --git a/tool/m4/colorize_result.m4 b/tool/m4/colorize_result.m4
index 19ff6d49e8..83912040e5 100644
--- a/tool/m4/colorize_result.m4
+++ b/tool/m4/colorize_result.m4
@@ -1,4 +1,4 @@
-# -*- Autoconf -*-
+dnl -*- Autoconf -*-
 AC_DEFUN([COLORIZE_RESULT], [AC_REQUIRE([_COLORIZE_RESULT_PREPARE])dnl
     AS_LITERAL_IF([$1],
 	[m4_case([$1],
diff --git a/tool/m4/ruby_append_option.m4 b/tool/m4/ruby_append_option.m4
index 07b6537cb7..ff828d2162 100644
--- a/tool/m4/ruby_append_option.m4
+++ b/tool/m4/ruby_append_option.m4
@@ -1,4 +1,4 @@
-# -*- Autoconf -*-
+dnl -*- Autoconf -*-
 AC_DEFUN([RUBY_APPEND_OPTION],
 	[# RUBY_APPEND_OPTION($1)
 	AS_CASE([" [$]{$1-} "],
diff --git a/tool/m4/ruby_append_options.m4 b/tool/m4/ruby_append_options.m4
index 164a4ab654..14213111ca 100644
--- a/tool/m4/ruby_append_options.m4
+++ b/tool/m4/ruby_append_options.m4
@@ -1,4 +1,4 @@
-# -*- Autoconf -*-
+dnl -*- Autoconf -*-
 AC_DEFUN([RUBY_APPEND_OPTIONS],
 	[# RUBY_APPEND_OPTIONS($1)
 	for rb_opt in $2; do
diff --git a/tool/m4/ruby_check_builtin_func.m4 b/tool/m4/ruby_check_builtin_func.m4
index 3fe5caf429..40abc78ef8 100644
--- a/tool/m4/ruby_check_builtin_func.m4
+++ b/tool/m4/ruby_check_builtin_func.m4
@@ -1,4 +1,4 @@
-# -*- Autoconf -*-
+dnl -*- Autoconf -*-
 AC_DEFUN([RUBY_CHECK_BUILTIN_FUNC], [dnl
 AC_CACHE_CHECK([for $1], AS_TR_SH(rb_cv_builtin_$1),
   [AC_LINK_IFELSE(
diff --git a/tool/m4/ruby_check_builtin_setjmp.m4 b/tool/m4/ruby_check_builtin_setjmp.m4
index 008fd45911..05118e2243 100644
--- a/tool/m4/ruby_check_builtin_setjmp.m4
+++ b/tool/m4/ruby_check_builtin_setjmp.m4
@@ -1,4 +1,4 @@
-# -*- Autoconf -*-
+dnl -*- Autoconf -*-
 AC_DEFUN([RUBY_CHECK_BUILTIN_SETJMP], [
 AS_IF([test x"${ac_cv_func___builtin_setjmp}" = xyes], [
    unset ac_cv_func___builtin_setjmp
diff --git a/tool/m4/ruby_check_printf_prefix.m4 b/tool/m4/ruby_check_printf_prefix.m4
index 0415f9fa92..15bb4aee87 100644
--- a/tool/m4/ruby_check_printf_prefix.m4
+++ b/tool/m4/ruby_check_printf_prefix.m4
@@ -1,4 +1,4 @@
-# -*- Autoconf -*-
+dnl -*- Autoconf -*-
 AC_DEFUN([RUBY_CHECK_PRINTF_PREFIX], [
 AC_CACHE_CHECK([for printf prefix for $1], [rb_cv_pri_prefix_]AS_TR_SH($1),[
     [rb_cv_pri_prefix_]AS_TR_SH($1)=[NONE]
diff --git a/tool/m4/ruby_check_setjmp.m4 b/tool/m4/ruby_check_setjmp.m4
index 66652984ea..6020b766b8 100644
--- a/tool/m4/ruby_check_setjmp.m4
+++ b/tool/m4/ruby_check_setjmp.m4
@@ -1,5 +1,5 @@
-# -*- Autoconf -*-
-# used for AC_ARG_WITH(setjmp-type)
+dnl -*- Autoconf -*-
+dnl used for AC_ARG_WITH(setjmp-type)
 AC_DEFUN([RUBY_CHECK_SETJMP], [
 AC_CACHE_CHECK([for ]$1[ as a macro or function], ac_cv_func_$1,
   [AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[
diff --git a/tool/m4/ruby_check_signedness.m4 b/tool/m4/ruby_check_signedness.m4
index 5376efa208..f9fbb3c088 100644
--- a/tool/m4/ruby_check_signedness.m4
+++ b/tool/m4/ruby_check_signedness.m4
@@ -1,4 +1,4 @@
-# -*- Autoconf -*-
+dnl -*- Autoconf -*-
 dnl RUBY_CHECK_SIGNEDNESS [typename] [if-signed] [if-unsigned] [included]
 AC_DEFUN([RUBY_CHECK_SIGNEDNESS], [dnl
     AC_COMPILE_IFELSE([AC_LANG_BOOL_COMPILE_TRY([AC_INCLUDES_DEFAULT([$4])], [($1)-1 > 0])],
diff --git a/tool/m4/ruby_check_sizeof.m4 b/tool/m4/ruby_check_sizeof.m4
index 38d7918f2e..975ac6c9be 100644
--- a/tool/m4/ruby_check_sizeof.m4
+++ b/tool/m4/ruby_check_sizeof.m4
@@ -1,4 +1,4 @@
-# -*- Autoconf -*-
+dnl -*- Autoconf -*-
 dnl RUBY_CHECK_SIZEOF [typename], [maybe same size types], [macros], [include]
 AC_DEFUN([RUBY_CHECK_SIZEOF],
 [dnl
diff --git a/tool/m4/ruby_check_sysconf.m4 b/tool/m4/ruby_check_sysconf.m4
index 8324be6764..f554786e77 100644
--- a/tool/m4/ruby_check_sysconf.m4
+++ b/tool/m4/ruby_check_sysconf.m4
@@ -1,4 +1,4 @@
-# -*- Autoconf -*-
+dnl -*- Autoconf -*-
 AC_DEFUN([RUBY_CHECK_SYSCONF], [dnl
 AC_CACHE_CHECK([whether _SC_$1 is supported], rb_cv_have_sc_[]m4_tolower($1),
   [AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[#include <unistd.h>
diff --git a/tool/m4/ruby_cppoutfile.m4 b/tool/m4/ruby_cppoutfile.m4
index 495ae0aae4..976cbb1c43 100644
--- a/tool/m4/ruby_cppoutfile.m4
+++ b/tool/m4/ruby_cppoutfile.m4
@@ -1,4 +1,4 @@
-# -*- Autoconf -*-
+dnl -*- Autoconf -*-
 AC_DEFUN([RUBY_CPPOUTFILE],
 [AC_CACHE_CHECK(whether ${CPP} accepts -o, rb_cv_cppoutfile,
 [save_CPPFLAGS="$CPPFLAGS"
diff --git a/tool/m4/ruby_decl_attribute.m4 b/tool/m4/ruby_decl_attribute.m4
index 22358a079a..a8a73dc870 100644
--- a/tool/m4/ruby_decl_attribute.m4
+++ b/tool/m4/ruby_decl_attribute.m4
@@ -1,4 +1,4 @@
-# -*- Autoconf -*-
+dnl -*- Autoconf -*-
 dnl RUBY_DECL_ATTRIBUTE(attrib, macroname, cachevar, condition, type, code)
 AC_DEFUN([RUBY_DECL_ATTRIBUTE], [dnl
 m4_ifval([$2], dnl
diff --git a/tool/m4/ruby_default_arch.m4 b/tool/m4/ruby_default_arch.m4
index a9e7177fda..03e52f7776 100644
--- a/tool/m4/ruby_default_arch.m4
+++ b/tool/m4/ruby_default_arch.m4
@@ -1,4 +1,4 @@
-# -*- Autoconf -*-
+dnl -*- Autoconf -*-
 AC_DEFUN([RUBY_DEFAULT_ARCH], [
 AC_MSG_CHECKING([arch option])
 AS_CASE([$1],
@@ -8,5 +8,4 @@ AS_CASE([$1],
 	[AC_MSG_ERROR(unknown target architecture: $target_archs)]
 	)
 AC_MSG_RESULT([$ARCH_FLAG])
-])
-dnl
+])dnl
diff --git a/tool/m4/ruby_define_if.m4 b/tool/m4/ruby_define_if.m4
index b18b104aad..1cfd07b0c8 100644
--- a/tool/m4/ruby_define_if.m4
+++ b/tool/m4/ruby_define_if.m4
@@ -1,4 +1,4 @@
-# -*- Autoconf -*-
+dnl -*- Autoconf -*-
 AC_DEFUN([RUBY_DEFINE_IF], [dnl
     m4_ifval([$1], [AS_LITERAL_IF([$1], [], [test "X$1" = X || ])cat <<EOH >> confdefs.h
 @%:@if $1
diff --git a/tool/m4/ruby_defint.m4 b/tool/m4/ruby_defint.m4
index e5b46a788f..e9ed68e5b8 100644
--- a/tool/m4/ruby_defint.m4
+++ b/tool/m4/ruby_defint.m4
@@ -1,4 +1,4 @@
-# -*- Autoconf -*-
+dnl -*- Autoconf -*-
 dnl RUBY_DEFINT TYPENAME, SIZE, [UNSIGNED], [INCLUDES = DEFAULT-INCLUDES]
 AC_DEFUN([RUBY_DEFINT], [dnl
 AS_VAR_PUSHDEF([cond], [rb_defint_cond])dnl
diff --git a/tool/m4/ruby_dtrace_available.m4 b/tool/m4/ruby_dtrace_available.m4
index babffaffac..e03b7762fc 100644
--- a/tool/m4/ruby_dtrace_available.m4
+++ b/tool/m4/ruby_dtrace_available.m4
@@ -1,4 +1,4 @@
-# -*- Autoconf -*-
+dnl -*- Autoconf -*-
 AC_DEFUN([RUBY_DTRACE_AVAILABLE],
 [AC_CACHE_CHECK(whether dtrace USDT is available, rb_cv_dtrace_available,
 [
diff --git a/tool/m4/ruby_dtrace_postprocess.m4 b/tool/m4/ruby_dtrace_postprocess.m4
index 1cb651b481..6fd6de7c9a 100644
--- a/tool/m4/ruby_dtrace_postprocess.m4
+++ b/tool/m4/ruby_dtrace_postprocess.m4
@@ -1,4 +1,4 @@
-# -*- Autoconf -*-
+dnl -*- Autoconf -*-
 AC_DEFUN([RUBY_DTRACE_POSTPROCESS],
 [AC_CACHE_CHECK(whether $DTRACE needs post processing, rb_cv_prog_dtrace_g,
 [
diff --git a/tool/m4/ruby_func_attribute.m4 b/tool/m4/ruby_func_attribute.m4
index b265fd41da..bce26fc16a 100644
--- a/tool/m4/ruby_func_attribute.m4
+++ b/tool/m4/ruby_func_attribute.m4
@@ -1,4 +1,4 @@
-# -*- Autoconf -*-
+dnl -*- Autoconf -*-
 dnl RUBY_FUNC_ATTRIBUTE(attrib, macroname, cachevar, condition)
 AC_DEFUN([RUBY_FUNC_ATTRIBUTE], [dnl
     RUBY_DECL_ATTRIBUTE([$1], [$2], [$3], [$4],
diff --git a/tool/m4/ruby_mingw32.m4 b/tool/m4/ruby_mingw32.m4
index 76b95f02a8..98e922340b 100644
--- a/tool/m4/ruby_mingw32.m4
+++ b/tool/m4/ruby_mingw32.m4
@@ -1,4 +1,4 @@
-# -*- Autoconf -*-
+dnl -*- Autoconf -*-
 AC_DEFUN([RUBY_MINGW32],
 [AS_CASE(["$host_os"],
 [cygwin*], [
diff --git a/tool/m4/ruby_prepend_option.m4 b/tool/m4/ruby_prepend_option.m4
index 3e868a6831..3b7030a473 100644
--- a/tool/m4/ruby_prepend_option.m4
+++ b/tool/m4/ruby_prepend_option.m4
@@ -1,4 +1,4 @@
-# -*- Autoconf -*-
+dnl -*- Autoconf -*-
 AC_DEFUN([RUBY_PREPEND_OPTION],
 	[# RUBY_PREPEND_OPTION($1)
 	AS_CASE([" [$]{$1-} "],
diff --git a/tool/m4/ruby_prog_gnu_ld.m4 b/tool/m4/ruby_prog_gnu_ld.m4
index 8d484f9440..b38fb3d527 100644
--- a/tool/m4/ruby_prog_gnu_ld.m4
+++ b/tool/m4/ruby_prog_gnu_ld.m4
@@ -1,4 +1,4 @@
-# -*- Autoconf -*-
+dnl -*- Autoconf -*-
 AC_DEFUN([RUBY_PROG_GNU_LD],
 [AC_CACHE_CHECK(whether the linker is GNU ld, rb_cv_prog_gnu_ld,
 [AS_IF([`$CC $CFLAGS $CPPFLAGS $LDFLAGS --print-prog-name=ld 2>&1` -v 2>&1 | grep "GNU ld" > /dev/null], [
diff --git a/tool/m4/ruby_replace_funcs.m4 b/tool/m4/ruby_replace_funcs.m4
index d0612e29a0..10e85f1ac9 100644
--- a/tool/m4/ruby_replace_funcs.m4
+++ b/tool/m4/ruby_replace_funcs.m4
@@ -1,13 +1,13 @@
-# -*- Autoconf -*-
+dnl -*- Autoconf -*-
 dnl RUBY_REPLACE_FUNC [func] [included]
 AC_DEFUN([RUBY_REPLACE_FUNC], [dnl
     AC_CHECK_DECL([$1],dnl
         [AC_DEFINE(AS_TR_CPP(HAVE_[$1]))],dnl
         [AC_REPLACE_FUNCS($1)],dnl
         [$2])dnl
-])
-
+])dnl
+dnl
 dnl RUBY_REPLACE_FUNCS [funcs] [included]
 AC_DEFUN([RUBY_REPLACE_FUNCS] [dnl
     m4_map_args_w([$1], [RUBY_REPLACE_FUNC(], [), [$2]])dnl
-])
+])dnl
diff --git a/tool/m4/ruby_replace_type.m4 b/tool/m4/ruby_replace_type.m4
index a7d2ffe938..70674b6cc7 100644
--- a/tool/m4/ruby_replace_type.m4
+++ b/tool/m4/ruby_replace_type.m4
@@ -1,4 +1,4 @@
-# -*- Autoconf -*-
+dnl -*- Autoconf -*-
 dnl RUBY_REPLACE_TYPE [typename] [default type] [macro type] [included]
 AC_DEFUN([RUBY_REPLACE_TYPE], [dnl
     AC_CHECK_TYPES([$1],
diff --git a/tool/m4/ruby_rm_recursive.m4 b/tool/m4/ruby_rm_recursive.m4
index e2c8f66ccc..b97701f88e 100644
--- a/tool/m4/ruby_rm_recursive.m4
+++ b/tool/m4/ruby_rm_recursive.m4
@@ -1,4 +1,4 @@
-# -*- Autoconf -*-
+dnl -*- Autoconf -*-
 AC_DEFUN([RUBY_RM_RECURSIVE], [dnl
 m4_version_prereq([2.70], [], [dnl
 # suppress error messages, rm: cannot remove 'conftest.dSYM', from
diff --git a/tool/m4/ruby_setjmp_type.m4 b/tool/m4/ruby_setjmp_type.m4
index 29893543a4..4ae26fe5cd 100644
--- a/tool/m4/ruby_setjmp_type.m4
+++ b/tool/m4/ruby_setjmp_type.m4
@@ -1,4 +1,4 @@
-# -*- Autoconf -*-
+dnl -*- Autoconf -*-
 AC_DEFUN([RUBY_SETJMP_TYPE], [
 RUBY_CHECK_BUILTIN_SETJMP
 RUBY_CHECK_SETJMP(_setjmpex, [], [@%:@include <setjmpex.h>])
diff --git a/tool/m4/ruby_stack_grow_direction.m4 b/tool/m4/ruby_stack_grow_direction.m4
index f5f93579a4..a4d205cc3c 100644
--- a/tool/m4/ruby_stack_grow_direction.m4
+++ b/tool/m4/ruby_stack_grow_direction.m4
@@ -1,4 +1,4 @@
-# -*- Autoconf -*-
+dnl -*- Autoconf -*-
 AC_DEFUN([RUBY_STACK_GROW_DIRECTION], [
     AS_VAR_PUSHDEF([stack_grow_dir], [rb_cv_stack_grow_dir_$1])
     AC_CACHE_CHECK(stack growing direction on $1, stack_grow_dir, [
diff --git a/tool/m4/ruby_try_cflags.m4 b/tool/m4/ruby_try_cflags.m4
index 8c9f22d50c..f2c6a3094e 100644
--- a/tool/m4/ruby_try_cflags.m4
+++ b/tool/m4/ruby_try_cflags.m4
@@ -1,4 +1,4 @@
-# -*- Autoconf -*-
+dnl -*- Autoconf -*-
 AC_DEFUN([RUBY_TRY_CFLAGS], [
     AC_MSG_CHECKING([whether ]$1[ is accepted as CFLAGS])
     RUBY_WERROR_FLAG([
diff --git a/tool/m4/ruby_try_cxxflags.m4 b/tool/m4/ruby_try_cxxflags.m4
index 265f79a450..06f645f546 100644
--- a/tool/m4/ruby_try_cxxflags.m4
+++ b/tool/m4/ruby_try_cxxflags.m4
@@ -1,4 +1,4 @@
-# -*- Autoconf -*-
+dnl -*- Autoconf -*-
 AC_DEFUN([RUBY_TRY_CXXFLAGS], [
     save_CXXFLAGS="$CXXFLAGS"
     CXXFLAGS="[$]CXXFLAGS $1"
diff --git a/tool/m4/ruby_try_ldflags.m4 b/tool/m4/ruby_try_ldflags.m4
index d27940c7b2..c3a6be0fb3 100644
--- a/tool/m4/ruby_try_ldflags.m4
+++ b/tool/m4/ruby_try_ldflags.m4
@@ -1,4 +1,4 @@
-# -*- Autoconf -*-
+dnl -*- Autoconf -*-
 AC_DEFUN([RUBY_TRY_LDFLAGS], [
     save_LDFLAGS="$LDFLAGS"
     LDFLAGS="[$]LDFLAGS $1"
diff --git a/tool/m4/ruby_type_attribute.m4 b/tool/m4/ruby_type_attribute.m4
index 6ddaa20c53..5ea1219c6e 100644
--- a/tool/m4/ruby_type_attribute.m4
+++ b/tool/m4/ruby_type_attribute.m4
@@ -1,4 +1,4 @@
-# -*- Autoconf -*-
+dnl -*- Autoconf -*-
 dnl RUBY_TYPE_ATTRIBUTE(attrib, macroname, cachevar, condition)
 AC_DEFUN([RUBY_TYPE_ATTRIBUTE], [dnl
     RUBY_DECL_ATTRIBUTE([$1], [$2], [$3], [$4],
diff --git a/tool/m4/ruby_universal_arch.m4 b/tool/m4/ruby_universal_arch.m4
index e2f698cbc0..a2ee9e02be 100644
--- a/tool/m4/ruby_universal_arch.m4
+++ b/tool/m4/ruby_universal_arch.m4
@@ -1,4 +1,4 @@
-# -*- Autoconf -*-
+dnl -*- Autoconf -*-
 AC_DEFUN([RUBY_UNIVERSAL_ARCH], [
 # RUBY_UNIVERSAL_ARCH begin
 ARCH_FLAG=`expr " $CXXFLAGS " : ['.* \(-m[0-9][0-9]*\) ']`
diff --git a/tool/m4/ruby_werror_flag.m4 b/tool/m4/ruby_werror_flag.m4
index e9155c8309..616a7f6abf 100644
--- a/tool/m4/ruby_werror_flag.m4
+++ b/tool/m4/ruby_werror_flag.m4
@@ -1,4 +1,4 @@
-# -*- Autoconf -*-
+dnl -*- Autoconf -*-
 AC_DEFUN([RUBY_WERROR_FLAG], [dnl
 save_CFLAGS="$CFLAGS"
 CFLAGS="$CFLAGS $rb_cv_warnflags"