Define WIN32 when _WIN32 is set

_WIN32 is set by the compiler, whereas our code uses WIN32 that is
normally set through our build system. To make it possible to build
extensions out of tree we cannot rely on that, so set the WIN32
symbol explicitly whenever the compiler has set _WIN32.

Not setting this symbol causes double inclusion of pg_config_os.h,
and possibly other errors as well.

Craig Ringer