Prefix lib files

A simple change to admin/cabal-config.in that implements the changes discussed in [issue 34](https://github.com/creswick/cabal-dev/issues/34).

This uncomments the libsubdir entry, thus adding a prefix of `$pkgid/$compiler` in order to avoid an interface file conflict when installing multiple packages that define the same module (reproduced with `cabal-dev install citeproc-hs`; citeproc-hs depends on packages using mtl and List, which both define Control.Monad.Trans.List).