Search for dune.module in private installations

When doing a private install of DUNE libraries, the dune.module file
is put in the lib/ directory, regardless of architecture.

This patch searches the lib/ directory as a catch-all after having
searched the platform-specific directories. This should minimize the
chance of hitting an accidentally unrelated dune.module.
Merge pull request #382 from rolk/382_dunemod

Search for dune.module in private installations