COMMON: Use true nullptr in Visual Studio 2010+ and true override in …

…VS 2012+

Those compilers support these features despite not being fully C++11-compliant.

<http://msdn.microsoft.com/en-us/library/hh567368.aspx> says that VS 2010
has "partial" support for override. I don't know what that entails and I
can't test it, so I err on the side of caution and only enable it in 2012
and up.

common/c++11-compat.h

@@ -32,13 +32,17 @@
 // though.
 //
 #if !defined(nullptr)  // XCode 5.0.1 has __cplusplus=199711 but defines this
+#if !defined(_MSC_VER) || _MSC_VER < 1600
 #define nullptr 0
 #endif
+#endif
 
 //
 // Replacement for the override keyword. This allows compilation of code
 // which uses it, but does not feature any semantic.
 //
+#if !defined(_MSC_VER) || _MSC_VER < 1700
 #define override
+#endif
 
 #endif