Specify root as the starting point for the linux compiler path

NOTE: New raylib release installer uses `w64devkit`, smaller and more updated than regular `mingw64-w64` default project. Paths are updated properly.

Now it works on Windows for me but there are lots of hardcoded paths around... I don't like it.

This requires the user to explicitly specify the path to their raylib folder. We need a better way around this (something like what we do for windows would help).