Update configuration-packages.nix files #70
Description
I ran into the same problem as reported in typeable/nixpkgs-stackage#38 (comment): lens failing to build with a "serious configure-time warning from Cabal " error.
I think the problem is that the configuration-packages.nix being generated by stackage2nix is not up-to-date; in particular one needs to add Cabal, stm, mtl, etc. to the list of core libraries to be used from GHC.
As a workaround for the lens, issue, mtl = null; as an override fixes it.