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.
diff --git a/include/json/writer.h b/include/json/writer.h
index 58c716b..2c1e65b 100644
--- a/include/json/writer.h
+++ b/include/json/writer.h
@@ -112,12 +112,12 @@
   Json::Value settings_;
 
   StreamWriterBuilder();
-  ~StreamWriterBuilder() override;
+  ~StreamWriterBuilder() JSONCPP_OVERRIDE;
 
   /**
    * \throw std::exception if something goes wrong (e.g. invalid settings)
    */
-  StreamWriter* newStreamWriter() const override;
+  StreamWriter* newStreamWriter() const JSONCPP_OVERRIDE;
 
   /** \return true if 'settings' are legal and consistent;
    *   otherwise, indicate bad settings via 'invalid'.
@@ -158,7 +158,7 @@
 
 public:
   FastWriter();
-  ~FastWriter() override {}
+  ~FastWriter() JSONCPP_OVERRIDE {}
 
   void enableYAMLCompatibility();
 
@@ -172,7 +172,7 @@
   void omitEndingLineFeed();
 
 public: // overridden from Writer
-  JSONCPP_STRING write(const Value& root) override;
+  JSONCPP_STRING write(const Value& root) JSONCPP_OVERRIDE;
 
 private:
   void writeValue(const Value& value);
@@ -210,14 +210,14 @@
 class JSON_API StyledWriter : public Writer {
 public:
   StyledWriter();
-  ~StyledWriter() override {}
+  ~StyledWriter() JSONCPP_OVERRIDE {}
 
 public: // overridden from Writer
   /** \brief Serialize a Value in <a HREF="http://www.json.org">JSON</a> format.
    * \param root Value to serialize.
    * \return String containing the JSON document that represents the root value.
    */
-  JSONCPP_STRING write(const Value& root) override;
+  JSONCPP_STRING write(const Value& root) JSONCPP_OVERRIDE;
 
 private:
   void writeValue(const Value& value);