Merge pull request #2411 in HDFFV/hdf5 from ~DYOUNG/werror:darwin-barriers to develop

* commit '803d805c74466a9d736455930b17de2d9f5cb02d':
  Complete the comment on thread_main(), explaining why the barrier is used.
  The first implementation seemed to allow for the possibility that a thread could block at the barrier, wake and exit the barrier, re-acquire the barrier lock and increase `nentered` before the other blocked threads woke and checked `nentered % count == 0`.  Then the other blocked threads would check `nentered % count == 0` and, finding it false, go back to sleep in the barrier.  This new implementation waits for a looser condition to obtain so that threads don't go back to sleep in the barrier.
  Test the right condition for the EBUSY return in pthread_barrier_destroy().
  s/exit_failure/EXIT_FAILURE/g
  Implement pthread_barrier(3) for Darwin using a counter, condition variable, and mutex.  Untested.

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