Do not duplicate handling of duplicate libraries

There were (at least) three implementations of the same macro; these
have been collected into a separate helper module. (Note that this
means that FindERT is now not completely stand-alone anymore).
Separate inclusion of OpenMP library into reusable module

Other projects (which does not have a direct dependency on ERT) require
OpenMP; we desire to reuse this functionality into those projects.
Propagate information about whether module was found quietly
Merge pull request #171 from rolk/171_openmp

Refactor OpenMP support to be reusable