SCons: fix #1672 Incorrect compiler detection in msystem based environments

[enetheru]

As described in issue #1672 msys2 environments don't use a mingw prefix for their compiler binaries.

This patch detects MSYSTEM environment variable detects compilers directly

[enetheru]

While this works for me, I dont think it is necessarily the best approach. If you are in an msystem environment shell it can reasonably be assumed you want to compile using whatever the compiler is within that shell. so passing use_mingw=yes or use_llvm=yes feels like a bad hacky holdover from previous code.

I think this might run afoul of needing the godot and godot-cpp build systems to be similar.

I quickly updated godot master and checked and within a msys2.clang64 environment it tries to use cl. and i need to pass use_mingw=yes, and then it detects clang. So same issues.

So to get this solved "properly" I think I have to go submit a proposal to godot to get it changed there too, and that will in turn flow down here?

@dsnopek what would you advise?