Handle new Cabal directory structure (fixes #2)

Cabal commit https://github.com/haskell/cabal/commit/fa448266af549336ab29db1978c47120de3deef7 changes the installation directory for libraries from:

    <libdir>/<libraryName>/<compiler>

to:

    <libdir>/<arch>-<os>-<compiler>/<libraryName>