Make the Windows #ifdef more universal

[squeek502]

• WIN32 is only defined by a few compilers, while _WIN32 is defined by all of them
• See: http://nadeausoftware.com/articles/2012/01/c_c_tip_how_use_compiler_predefined_macros_detect_operating_system#WindowsCygwinnonPOSIXandMinGW

[dsuni]

As you may have noticed, I'm not a Windows guy myself. The original code was a contribution as well. Will merge it. Thanks.