Avoid spurious warnings that made the "long long int is 64 bits" test fail

--- a/config/c-compiler.m4
+++ b/config/c-compiler.m4
@@ -49,8 +49,9 @@ int does_int64_work()
     return 0;
   return 1;
 }
+int
 main() {
-  exit(! does_int64_work());
+  return(! does_int64_work());
 }],
 [Ac_cachevar=yes],
 [Ac_cachevar=no],
