Turn more licenses into lib.licenses style

Should eval cleanly, as far as -A tarball tells me.

Relevant: issue #2999, issue #739

pkgs/tools/graphics/povray/default.nix

@@ -33,10 +33,10 @@ stdenv.mkDerivation {
       PATH="$TMP/bin:$PATH"
     done
   '';
-  
+
   meta = {
     homepage = http://www.povray.org/;
     description = "Persistence of Vision Raytracer";
-    license = "free";
+    license = stdenv.lib.licenses.free;
   };
 }