Eliminate all usage of __WIN32__ macro: Some compilers (e.g. Clang/LLVM) don't define it, and _WIN32 is much more portable anyway. 
See: [http://nadeausoftware.com/articles/2012/01/c_c_tip_how_use_compiler_predefined_macros_detect_operating_system#WindowsCygwinnonPOSIXandMinGW]