Change library search path on MacOS

When using mac ports GL/gl.h ends up outside the cmake search paths for
libraries and includes. By adding them to mk/macos/CMakeLists.txt they are now
available and make it possible to build g3d_viewer along with the rest of
megaglest. To make this work properly I had to move the inclusion of
mk/macos/CMakeLists.txt further up the master CMakeLists.txt so merged both IF
APPLE tests in to one.

This has built successfully through xcode and gnumake.
Remove unneeded comments

These comments indicated TODO items and are no longer needed in the final change.
Merge pull request #39 from goetzk/change-library-search-paths-on-osx

Change library search paths on osx