gnustep: cleanup

Cleaning up. Adding GNUstep package builder for abstracting out GNUstep
compilation specifics (with thanks to GitHub user lethalman).

The rules for using build_gnustep_package are as simple: any
GNUstep-based package that the package being compiled depends upon are
to be put in [deps] (this is used for setting up a buildEnv), while
other dependencies are put into [buildInputs] as usual.