Probe explicitly for OpenMP

opm-core contains code which has #pragma omp. Earlier, OpenMP was
detected as part of the probe for ERT. However, that probe got
smarter, and as a result doesn't probe for more than it needs. Thus,
we need to check for OpenMP explicitly in opm-core. (It is disabled
by default)
Merge pull request #181 from rolk/182_openmp

Probe explicitly for OpenMP