Set -std=gnu99 for PHP extension compile.

php/ext/google/protobuf/config.m4

@@ -5,6 +5,6 @@ if test "$PHP_PROTOBUF" != "no"; then
   PHP_NEW_EXTENSION(
     protobuf,
     arena.c array.c convert.c def.c map.c message.c names.c php-upb.c protobuf.c,
-    $ext_shared)
+    $ext_shared, , -std=gnu99)
 
 fi