Use macro for override

b/c MS VS2010 is supposed to be C++11 but does not fulfull
the entire standard.

Resolves #410.
Re: #430.

src/lib_json/json_reader.cpp

@@ -1907,7 +1907,7 @@ public:
   {}
   bool parse(
       char const* beginDoc, char const* endDoc,
-      Value* root, JSONCPP_STRING* errs) override {
+      Value* root, JSONCPP_STRING* errs) JSONCPP_OVERRIDE {
     bool ok = reader_.parse(beginDoc, endDoc, *root, collectComments_);
     if (errs) {
       *errs = reader_.getFormattedErrorMessages();