Minor cleanups to bli_threading.h and friends.

Details:
- No longer need to define BLIS_ENABLE_MULTITHREADING manually in
  bli_config.h; it now gets defined when BLIS_ENABLE_OPENMP or
  BLIS_ENABLE_PTHREADS is defined.
- Added sanity check to prevent both BLIS__ENABLE_OPENMP and
  BLIS_ENABLE_PTHREADS from being enabled simultaneously.
- Reorganization of bli_threading*.h header files, which led to
  simplification of threading-related part of blis.h.
- added "-fopenmp -lpthread" to LDFLAGS of sandybridge make_defs.mk
  file.
Merge branch 'master' of github.com:flame/blis