Make glu a regular dependency

While packaging a program that uses freeglut we realized that to build a package using freeglut (using GL/glut.h) one needs to also have GL/glu.h which is provided by the glu package. This package however is just a makedepend so it is possible to not have it installed.

https://github.com/ocaml/opam-repository/pull/28840#issuecomment-3491263738