Windows XP toolchain fix from Bruce Dawson:
Fix to allow using SDL when compiling with v110_xp or v120_xp -- compiling with VS2012/VS2013 with the XP targeting option.

In order to ensure that we can target Windows XP we compile with the v120_xp toolset instead of v120. This means that we use an earlier SDK version and it means that winapifamily.h is not available. Compiling for this old SDK can be detected using the _USING_V110_SDK71_ define which is set through the %(PreprocessorDefinitions) option.