windows: use built-in _WIN32 macro to detect Windows

Instead of `WIN32`.

The compiler defines `_WIN32`. Windows SDK headers or build env defines
`WIN32`, or we have to take care of it. The agreement seems to be that
`_WIN32` is the preferred practice here.

Minor downside is that CMake uses `WIN32` and we also adopted it in
`Makefile.mk`.

In public libssh2 headers we stick with accepting either `_WIN32` or
`WIN32` and define our own namespaced `LIBSSH2_WIN32` based on them.

grepping for `WIN32` remains useful to detect Windows-specific code.

Closes #1195

tests/test_read.c

@@ -38,7 +38,7 @@ int test(LIBSSH2_SESSION *session)
     /* Ignore our hard-wired Dockerfile user when not running under Docker */
     if(!openssh_fixture_have_docker()) {
         username = getenv("USER");
-#ifdef WIN32
+#ifdef _WIN32
         if(!username)
             username = getenv("USERNAME");
 #endif