Added support for overridable variants of package.

For details read comments near getVersion function, gettext is the current
example.

svn path=/nixpkgs/trunk/; revision=9235

pkgs/development/libraries/gettext/default.nix

@@ -1 +1,8 @@
-import ./gettext-0.14.6.nix
+args:
+rec {
+	recurseForDerivations = true;
+	default = v_0_14_6;
+	v_0_14_6 = (import ./0.14.6.nix) args;
+	v_0_15 = (import ./0.15.nix) args;
+	v_0_16_x = (import ./0.16.x.nix) args;
+}