FIX: FindVGlut now prefers freeglut over glut in all occasions

FIX: FindVGlut properly re-searches GLUT_LIBRARIES if variable is cleared

git-svn-id: https://svn.rwth-aachen.de/repos/vrgroup-svn/projects/VistaCMakeCommon/trunk@30494 33c38b33-09d2-4561-8d4d-1bfa2f58f885