GNUstep packages for NixOS

I would like NixOS to support repeatable builds for GNUstep, since obtaining up-to-date GS on Debian/Ubuntu is quite a bit of pain.

Currently, Gorm and ProjectCenter applications compile and mostly work (but only if environment is set up properly), and I'm figuring out the way forward from here.

I'll keep updating this PR as I move along.

For now, I'd like to create some build support for building GS-based applications. The issue is that GS by default (a) hard-codes some vital paths, and (b) tries to write to locations in the gnustep-make package (as a consequence of (a)). I think that it should be possible to generate a gnustep conf file containing all the necessary paths and then sourcing GNUstep.sh prior to building, which should simplify things considerably.

Also note that I haven't tested if it will work under Darwin, yet (despite what the platforms metadata will tell).