[CMake] Equivalent of -T v120_xp with ninja generator

[Thomas Sondergaard]

Hi,

On Windows we use -G "Visual Studio 12" with -T v120_xp, thus using 
Visual Studio 2013 to build binaries that are compatible with Windows 
XP. It would be a great help if someone could tell me how to accomplish 
the same with the Ninja generator. I'd like to switch to using Ninja on 
Windows as our build is significantly faster with Ninja.

Thanks,
Thomas