mesa: use buildEnv to join mesa(noglu) and glu

We had a brief discussion in https://github.com/NixOS/nixpkgs/commit/aa0fb7306430be2222aa89b88a060f80d7a023c0 and @peti suggested what I think was the best solution to the upstream split of mesa and glu.

From the (little) tests I did, it seems to work (for example, SDL GL apps do work again) and I found no more references of mesa_noglu in nixpkgs besides the wxGTK one that I already fixed.