configure.ac: do not overwrite -std=

Set -std=gnu99 only when no such compiler flag(s) are set in any
compiler-related environment variables.  This enables users to
specify something more modern, say, -std=c11.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@65154 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

configure.ac

@@ -626,6 +626,9 @@ AS_IF([test "$GCC" = yes], [
     ],
     [
       # ANSI (no XCFLAGS because this is C only)
+      rb_tmp_std_check=`echo $CC $CFLAGS $optflags $warnflags $debugflags | fgrep std= | tr -d  '\015'`
+      AS_IF([test "x$rb_tmp_std_check" = "x"],
+      [
 	for ansi_options in -std=gnu99; do
 	    RUBY_TRY_CFLAGS(${ansi_options}, [
 		RUBY_APPEND_OPTIONS(warnflags, ${ansi_options})
@@ -633,6 +636,7 @@ AS_IF([test "$GCC" = yes], [
 	    ], [ansi_options=])
 	    test "x${ansi_options}" = x || break
 	done
+      ])
     ])
 
     # suppress annoying -Wstrict-overflow warnings