Check __STDC_VERSION__ is defined before checking its value

Prevent an undef warning regarding __STDC_VERSION__ by checking whether it is defined before checking its value.

json_object.h

@@ -452,7 +452,7 @@ JSON_EXPORT void json_object_object_del(struct json_object *obj, const char *key
  * @param val the local name for the json_object* object variable defined in
  *            the body
  */
-#if defined(__GNUC__) && !defined(__STRICT_ANSI__) && __STDC_VERSION__ >= 199901L
+#if defined(__GNUC__) && !defined(__STRICT_ANSI__) && (defined(__STDC_VERSION__) && __STDC_VERSION__ >= 199901L)
 
 #define json_object_object_foreach(obj, key, val)                                \
 	char *key = NULL;                                                        \
@@ -484,7 +484,7 @@ JSON_EXPORT void json_object_object_del(struct json_object *obj, const char *key
 	                 : 0);                                                 \
 	     entry##key = entry_next##key)
 
-#endif /* defined(__GNUC__) && !defined(__STRICT_ANSI__) && __STDC_VERSION__ >= 199901L */
+#endif /* defined(__GNUC__) && !defined(__STRICT_ANSI__) && (defined(__STDC_VERSION__) && __STDC_VERSION__ >= 199901L) */
 
 /** Iterate through all keys and values of an object (ANSI C Safe)
  * @param obj the json_object instance