[CMake] Colored diagnostic output for GCC 4.9 through Ninja

[Robert Dailey]

I'm using CMake + Ninja against GCC 4.9 in the Android NDK. I'm on
Windows, invoking 'ninja' through CMD.exe.

I do not see colored output for diagnostics (errors, warnings, etc)
GCC gives. Is there something in CMake I need to configure to enable
colored output?