Don't load external GLSL files by default

External GLSL should probably only be used for development testing,
not released products. The GLSL files are tied to the code, and the
code changes some what often.

Fixes using OpenArena 0.8.8 which has incompatible GLSL files in a pk3.
Don't use -mwindows with Clang on Windows

Thanks to stigmha for pointing out this doesn't work in Windows.
Merge remote-tracking branch 'ioquake3/master'