FindOpenGL: Add support for a separate GLU include directory
Add a OPENGL_GLU_INCLUDE_DIR cache variable for GL/glu.h
and attach it
as an include directory to the OpenGL::GLU
target. Also add a
OPENGL_INCLUDE_DIRS
result variable to list all include directories.
Account for the GLU header when deciding whether GLU is found.
Fixes: #25627 (closed)
Edited by Brad King