Fix Windows specific macro checks

[bodand]

The Windows macro checks check for _WINDOWS to be defined, however, as I checked it is not defined on any compiler I have installed (GCC 9.2; clang 8.0.1; MSVC 19.21.27702.2) that can target Windows. It is not even mentioned on MSDN.
To note, my setup couldn't compile the program as is; it thought I was on something non-windows, and tried to include <termios.h>.

Momentarily, I can do a pull request, to patch this, if it's alright.

Appendix
GCC & clang check:

MSVC check: