Get rid of annoying build warnings

When building Wix with only VS2013 installed, the CompileNativeResources task generates the following warnings:

    C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\include\sal.h(2886): warning RC4005: '__useHeader' : redefinition
    C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\include\sal.h(2896): warning RC4005: '__on_failure' : redefinition

Get rid of them by passing in "/D \_USING_V110_SDK71\_" to rc.exe when using the v7.1A Platform SDK and msbuild >= 12.0.