Fixed|Windows: Missing exports and OpenGLGL API callback
libgui|Cleanup: Removed ARB_debug_output

This extension wasn't in use and seems to be subtly differently defined
on different platforms.
Cleanup|qmake|64-bit: Preprocessor symbol for 64-bit builds

Conforming to naming conventions.
Merge remote-tracking branch 'remotes/origin/master' into savegame-refactor
libcommon: Cleanup