[build] simplify OSX build environment vars
[build] simplify OSX build environment vars

Since we fully build our own GTK/Mono stack, the build environment
variables are much more straightforward.

BansheeMediaPlayer/bockbuild,bl8/bockbuild,bl8/bockbuild,mono/bockbuild,mono/bockbuild,BansheeMediaPlayer/bockbuild,BansheeMediaPlayer/bockbuild,bl8/bockbuild