Implement initial Wayland support

This commit creates a new "wayland" backend, which can be
activated at configure time with "-DFREEGLUT_WAYLAND=ON".

If done so, it will be used instead of X11 (building both
and doing runtime detection may become possible later).
Please note that if you choose to use GL instead of GLES
(by not specifying "-DFREEGLUT_GLES=ON"), then libX11
will still be pulled as an indirect dependency.

Following features are still WIP :
- menus (not implemented, TODO) ;
- client-side decorations (not implemented, required
  because Wayland shells do not draw title bars nor
  resize grips, TODO) ;
- game mode (code is commented out, depends on WIP
  protocols tested upstream, WAIT FOR UPSTREAM) ;
- window visibility states (code is commented out,
  depends on xdg-shell protocol, TODO).

Signed-off-by: Manuel Bachmann <tarnyko@tarnyko.net>

git-svn-id: svn+ssh://svn.code.sf.net/p/freeglut/code/trunk/freeglut/freeglut@1758 7f0cb862-5218-0410-a997-914c9d46530a