faac is redistributable, hydra is allowed to build it.

pkgs/development/libraries/faac/default.nix

···
     homepage = http://www.audiocoding.com/faac.html;
     # Incompatible with GPL. Some changes to the base code, included in faac,
     # are under LGPL though.
-    license = "unfree";
+    license = "unfree-redistributable";
   };
 }