diff --git a/ChangeLog b/ChangeLog
index b95e24657e..2ecf03c9d1 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Wed Jun 13 17:51:05 2012  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* configure.in: cygwin needs C99 for some stuff, e.g.,
+	  pthread_attr_setstacksize, sched_yield.
+
 Wed Jun 13 17:50:43 2012  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* Makefile.in (.c.i): add warnflags to make the result consistent with
diff --git a/configure.in b/configure.in
index ab53e8a2c2..e94b3d6309 100644
--- a/configure.in
+++ b/configure.in
@@ -617,6 +617,11 @@ if test "$GCC" = yes; then
       # comments.   We  bypass  ANSI   C  mode  for  them.   Otherwise
       # extension libs cannot include those headers.
     ],
+    [cygwin*], [
+      # needs C99 to isinf() and so on
+        RUBY_APPEND_OPTION(warnflags, -ansi -std=c99)
+        RUBY_APPEND_OPTION(rb_cv_warnflags, -ansi -std=c99)
+    ],
     [
       # ANSI (no XCFLAGS because this is C only)
       RUBY_TRY_CFLAGS(-ansi -std=iso9899:199409, [