Removed nonstandard #warning constructs. Converted to XXX comments.

Removed nonstandard #warning constructs.  Converted to XXX comments.
(These are warnings about gamemode features not working the same on
some systems as on others.  The semantics of gamemode are dubious,
since it was never formally documented, and has for a long time
worked in different ways on different systems, so it is arguable that
there is nothing really wrong with systems that don't support
resolution-changes.)

git-svn-id: svn+ssh://svn.code.sf.net/p/freeglut/code/trunk/freeglut/freeglut@468 7f0cb862-5218-0410-a997-914c9d46530a