This macro was only used in test/example code before, now it is
also used in library code, but only defined automatically by
automake/cmake, so let's do the same for the standalone win32
make file.
It'd be probably better to just rely on the built-in _WIN32 macro
to detect the presence of windows.h though. It's already used
in most of libssh2 library code. There is a 3rd, similar macro
named LIBSSH2_WIN32, which might also be replaced with _WIN32.
Ref: 8b870ad771
Closes https://github.com/libssh2/libssh2/pull/266
64769acab5