Consistently use _WIN32 instead of __WIN32__

Consistently use _WIN32 instead of __WIN32__

_WIN32 works with all compilers, while __WIN32__ is MinGW-specific.
This project used __WIN32__ in files that only support MinGW, and _WIN32
in other files such as the library header and example programs.  One
place even used WIN32.  Avoid this unnecessary complication by just
always using _WIN32.