[Nix-dev] some fixes for multiple-outputs

[Mathijs Kwik]

Hi all,

I tried to build a next-generation system:
- stdenv-fixes
- x-updates
- multiple-outputs

I got quite far, most server-stuff (LAMP) builds and after some
patching/fixing, most GUI stuff builds too (gtk and qt at least).

I needed to do a few fixes, but I'm not confident they are the best solution.
Also, I won't be able to continue hacking this branch for some time,
so I'll just share my commits as some kind of "cherry-pick request" :)

https://github.com/bluescreen303/nixpkgs/commits/multiple-outputs

I hope this helps.
Mathijs