Check if _FORTIFY_SOURCE really works

i686-pc-cygwin gcc 6.4.0 seems broken around ssp.h, when compiling
with both of optimization and _FORTIFY_SOURCE.

configure.ac

@@ -673,7 +673,8 @@ AS_IF([test "$GCC" = yes], [
 				 [disable -D_FORTIFY_SOURCE=2 option, which causes link error on mingw]),
 		  [fortify_source=$enableval])
     AS_IF([test "x$fortify_source" != xno], [
-    RUBY_TRY_CFLAGS(-D_FORTIFY_SOURCE=2, [RUBY_APPEND_OPTION(XCFLAGS, -D_FORTIFY_SOURCE=2)])
+        RUBY_TRY_CFLAGS([$optflags -D_FORTIFY_SOURCE=2], [RUBY_APPEND_OPTION(XCFLAGS, -D_FORTIFY_SOURCE=2)], [],
+                        [@%:@include <stdio.h>])
     ])
 
     : ${MJIT_HEADER_FLAGS='-P -dD'}