Add a new way to handle option sets.

svn path=/nixpkgs/trunk/; revision=12505

pkgs/test/mkOption/keep.nix

@@ -0,0 +1,11 @@
+let
+  pkgs = import ../../top-level/all-packages.nix {};
+  config = import ./declare.nix;
+in
+  with (pkgs.lib);
+
+  finalReferenceOptionSets
+    filterOptionSets
+    pkgs
+    # List of main configurations.
+    [ config.configB config.configC ]