Enables usage of override specifier for MSVC compilers (beginning with 17.0...

Enables usage of override specifier for MSVC compilers (beginning with 17.0 which is the compiler of Visual Studio 2012)

src/pugixml.hpp

@@ -85,6 +85,8 @@
 #ifndef PUGIXML_OVERRIDE
 #	if __cplusplus >= 201103
 #		define PUGIXML_OVERRIDE override
+#	elif defined(_MSC_VER) && _MSC_VER >= 1700
+#		define PUGIXML_OVERRIDE override
 #	else
 #		define PUGIXML_OVERRIDE
 #	endif