Bugfix wrong variable used

But since it added the option anyway, this was hard to catch (unless
one was trying to set both options at the same time)
Disable use of OpenMP by default

If support for OpenMP should be added, it must be specified by
setting an option. If not, then warnings for unknown pragmas are
disabled, so that the code can have #pragma omp witout getting
messages during build.
Merge pull request #178 from rolk/178_openmp

Require explicit enabling of experimental OpenMP support