* Added an option `nixpkgs.config' that allows the Nixpkgs options to

  be set from the NixOS configuration.  For instance, you can say

    nixpkgs.config.firefox.enableGeckoMediaPlayer = true;

    environment.systemPackages = [ pkgs.firefox ];

  but the more interesting application is to apply global overrides to
  Nixpkgs throughout NixOS, e.g.

    nixpkgs.config.packageOverrides = pkgs:
      { glibc = pkgs.glibc27;
        gcc = pkgs.gcc42;
      };

  would build the whole system with Glibc 2.7 and GCC 4.2.  (There are
  some issues with "useFromStdenv" in all-packages.nix that need to be
  fixed for packages in the stdenv bootstrap though.)

  The implementation of this option is kind of evil though due to the
  need to prevent a circularity between the evaluation of
  nixpkgs.config and the "pkgs" module argument.

svn path=/nixos/trunk/; revision=16866