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
The OLD behaviors of all policies are deprecated, but only by
documentation. Add an explicit deprecation diagnostic for policies
introduced in CMake 3.11 and below to encourage projects to port away
from setting policies to OLD.
