missing.h: __STDC_WANT_LIB_EXT1__

* include/ruby/missing.h: get rid of redefinition of
  __STDC_WANT_LIB_EXT1__.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@52863 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

include/ruby/missing.h

@@ -18,7 +18,9 @@ extern "C" {
 #endif
 #endif
 
+#ifndef __STDC_WANT_LIB_EXT1__
 #define __STDC_WANT_LIB_EXT1__ 1
+#endif
 
 #include "ruby/config.h"
 #include <stddef.h>