fix detection of mbstate_t with clang

AC_TRY_COMPILE expexts a function body

Author: m6w6

ext/standard/config.m4

@@ -575,7 +575,7 @@ AC_TRY_COMPILE([
 # include <wchar.h>
 #endif
 ],[
-int __tmp__() { mbstate_t a; }
+mbstate_t a;
 ],[
   ac_cv_type_mbstate_t=yes
 ],[