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