Add config option ‘allowUnfree’

If set to false, mkDerivation will throw an exception if a package has
an unfree license.  ‘release-lib.nix’ uses this to enforce that we
don't build unfree packages as part of the Nixpkgs channel.  Since
this is set through Nixpkgs' ‘config’ argument, it's more finegrained
than $HYDRA_DISALLOW_UNFREE.

pkgs/os-specific/linux/aufs/3.nix

@@ -33,6 +33,7 @@ stdenv.mkDerivation {
     '';
 
   passthru = { inherit patch; };
+
   meta = {
     description = "Another Unionfs implementation for Linux (third generation)";
     homepage = http://aufs.sourceforge.net/;

pkgs/stdenv/generic/default.nix

@@ -10,7 +10,7 @@ let
 
   lib = import ../../lib;
 
-  disallowUnfree = builtins.getEnv "HYDRA_DISALLOW_UNFREE" == "1";
+  allowUnfree = config.allowUnfree or true && builtins.getEnv "HYDRA_DISALLOW_UNFREE" != "1";
 
   stdenvGenerator = setupScript: rec {
 
@@ -41,7 +41,7 @@ let
         # Add a utility function to produce derivations that use this
         # stdenv and its shell.
         mkDerivation = attrs:
-          if disallowUnfree && attrs.meta.license or "" == "unfree" then
+          if !allowUnfree && attrs.meta.license or "" == "unfree" then
             throw "package ‘${attrs.name}’ has an unfree license, refusing to evaluate"
           else
           (derivation (

pkgs/top-level/release-lib.nix

@@ -1,7 +1,11 @@
 rec {
-  allPackages = import ./all-packages.nix;
 
-  pkgs = allPackages {};
+  # Ensure that we don't build packages marked as unfree.
+  allPackages = args: import ./all-packages.nix (args // {
+    config.allowUnfree = false;
+  });
+
+  pkgs = allPackages { };
 
   /* !!! Hack: poor man's memoisation function.  Necessary for prevent
      Nixpkgs from being evaluated again and again for every