setup.py: Added a command-line option to omit extra include dirs (/usr/local/include), which would cause problems on some systems (e.g. BuildEM)
segmentation.hxx: Don't include <omp.h> if CYLEMON_NO_OPENMP is defined.
(clang does not support OpenMP yet.)
setup.py: Added a command-line arg --no-openmp, which disables OpenMP in segmentation.hxx