Optional glbinding as OpenGL binding

This week my team published an alternative OpenGL binding beside GLEW.
As a simple test we tried to replace the OpenGL binding of known open-source software with ours: glbinding.

For SuperTux this was a pleasingly simple task.
I just want you to be able to use our work we have done while integrating glbinding.
If you pull this PR into your branch, the build defaults to the exact same setting as without this PR.

That has been done:
 * add glbinding as optional dependency
 * add an option that glbinding can be used instead of GLEW
 * add another option to enable per OpenGL-function tracing during runtime
 * adapt OpenGL-dependent code so that it works with both GLEW and glbinding

I know this is just a mirror, but if you're interested in the code you can merge this PR in your internally used git-Repository.
Alright, but they should be the same in line 75 and line 84, so that this actually works, right? In any case: Many thanks for your work.