Applied Christian's change to drop unnecessary dependency to X11 libraries we

don't actually use, by asking cmake to link with X11_X11_LIB instead of the
catch-all X11_LIBRARIES.

git-svn-id: svn+ssh://svn.code.sf.net/p/freeglut/code/trunk/freeglut/freeglut@1845 7f0cb862-5218-0410-a997-914c9d46530a