Use more environment variables for OS detection

[Brycey92]

I replaced a ton of preprocessor OS detections with more robust ones. However, I didn't replace all of them, especially where it looked like the author was intentionally not using all possible environment variable names (that they knew of) to detect a certain OS.

As a warning, I've only tested this in a Windows Cygwin environment (by compiling the Swadge repo) so other environments and OSes still need to be tested.