cmake: make glut a soft dependency
There's parts of the tree that is useful even without GLUT, so let's allow building those without requiring GLUT. But let's emit a warning so users have a chance to understand why they get such a stripped-down build.
This is especially useful on Windows, where GLUT is a bit of an exotic dependency.
This gets us a bit closer to what we're doing with autotools.
Edited by Erik Faye-Lund