configure.ac: separate -std=gnu99 condition

Also necessary on mingw, cygwin, darwin and netbsd.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@67002 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

configure.ac

@@ -615,9 +615,11 @@ AS_IF([test "$GCC" = yes], [
     ],
     [cygwin*|darwin*|netbsd*], [
       # need lgamma_r(), finite()
-    ],
-    [solaris*], [
+    ])
+
     # ANSI (no XCFLAGS because this is C only)
+    AS_CASE(["$target_os"],
+    [solaris*], [
       # Because "-std=gnu99" affects existance of functions on Solaris,
       # "-std=gnu99" will be appended to CPPFLAGS.
 	for ansi_options in -std=gnu99; do