RF: don't use -fopenmp flag if it doesn't work

Fairly general refactor of setup to try candidate compile flags, and
remove flags from compile that cause a compile or link error.

MinRK's pyzmq zeromq build machinery was the main inspiration for this
way of doing it.
Merge pull request #371 from matthew-brett/check-openmp-flags

RF: don't use -fopenmp flag if it doesn't work