try to find glut library and include files

GLUT_INCLUDE_DIR, where to find GL/glut.h, etc.
GLUT_LIBRARIES, the libraries to link against
GLUT_FOUND, If false, do not try to use GLUT.

Also defined, but not for general use are:

GLUT_glut_LIBRARY = the full path to the glut library.
GLUT_Xmu_LIBRARY  = the full path to the Xmu library.
GLUT_Xi_LIBRARY   = the full path to the Xi Library.