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/libmng/default.nix

@@ -13,7 +13,7 @@ stdenv.mkDerivation rec {
   meta = {
     description = "Reference library for reading, displaying, writing and examining Multiple-Image Network Graphics";
     homepage = http://www.libmng.com;
-    license = "zlib/libpng";
+    license = stdenv.lib.licenses.zlib;
     maintainers = with stdenv.lib.maintainers; [ marcweber urkud ];
     hydraPlatforms = stdenv.lib.platforms.linux;
   };