MinGW defines both _WIN32 and WIN32 (and may even be the only compiler
doing so). Microsoft and Intel compilers only define _WIN32. Use the
common one to eliminate the need in defining WIN32 explicitly.
Same macro name is used in both natpmp and miniupnp libraries so it's
impossble to link one statically and another dynamically. This patch
renames the macro to have library-specific name.
Thomas Bernard
Mike Gelfand