Add special variable for MultiArch-unaware components

Some components are not yet MultiArch-aware and installs to either
lib/ (Debian) or lib64/ (RHEL) on 64-bit platforms. If we need to
interface with these components, then it is nice to have such a
variable set together with the regular detection code.
Use MultiArch-unaware path for dune.module

As of 2013-09 the DUNE code that reads dune.module is not MultiArch-
aware. Thus, for a 64-bit platform it will look in lib/ on Debian and
lib64/ on RHEL.
Merge pull request #373 from rolk/373_dunemod

Make finding and installing dune.module work on both Debian and RHEL simultaneously