Use _WIN32 as that it defined by the visual studio compilers.

WIN32 is defined by the CMake visual studio generators, and isn't present
when using ninja. Instead we use _WIN32 which is defined by the visual
studio compiler tool chain.

Change-Id: I3634a2def2da7094c2d9b62c0762da3397f92b97
Merge topic 'win32_compile_flag' into master

16d68ca7 Use _WIN32 as that it defined by the visual studio compilers.