Turn more licenses into lib.licenses style

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

Relevant: issue #2999, issue #739

pkgs/development/libraries/qhull/default.nix

@@ -15,6 +15,6 @@ stdenv.mkDerivation rec {
   meta = {
     homepage = http://www.qhull.org/;
     description = "Computes the convex hull, Delaunay triangulation, Voronoi diagram and more";
-    license = "free";
+    license = stdenv.lib.licenses.free;
   };
 }