proc.c: suppress warning

* proc.c (NO_CLOBBERED): suppress clobbered warning which appears
  only on mingw.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@56852 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

proc.c

@@ -18,7 +18,7 @@
  * versions */
 #define PROC_NEW_REQUIRES_BLOCK 0
 
-#if !defined(__GNUC__) || __GNUC__ < 5
+#if !defined(__GNUC__) || __GNUC__ < 5 || defined(__MINGW32__)
 # define NO_CLOBBERED(v) (*(volatile VALUE *)&(v))
 #else
 # define NO_CLOBBERED(v) (v)