Jordan Williams authored Feb 02, 2024 and Brad King committed Feb 05, 2024

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