build system now supports more build environments

[BasisDeveloper]

Before if you were compiling with Clang on Windows using MSYS2 or any environment that wasn't Windows native it would try and use MSVC build flags no matter what compiler was selected. I fixed that issue with my edits. Change anything you dislike.
^{(By the way, I've never contributed to someone else's project, if I'm suppose to open an issue first no matter what, I'm genuinely sorry)}

[maleadt]

Can confirm that this is needed to fix:

[12:08:15] i686-w64-mingw32-g++: error: /Zc:inline: No such file or directory
[12:08:15] i686-w64-mingw32-g++: error: /EHs-c-: No such file or directory
[12:08:15] i686-w64-mingw32-g++: error: /GR-: No such file or directory
[12:08:15] i686-w64-mingw32-g++: error: /wd4244: No such file or directory
[12:08:15] i686-w64-mingw32-g++: error: /wd4267: No such file or directory
[12:08:15] make[2]: *** [CMakeFiles/demumble.dir/build.make:77: CMakeFiles/demumble.dir/demumble.cc.obj] Error 1
[12:08:15] make[2]: *** Waiting for unfinished jobs....

@nico Could this be merged?