Most OSs have the `pthread_barrier' class of functions. But on those that don't have this optional Pthreads class, we have included our own implementation in `libs/threads.c'.

Our implementation can still need the `pthread_barrier' structure after `pthread_barrier_destroy' is called: the `pthread_cond_wait' loop in the `pthread_barrier_wait' can finish after `pthread_barrier_destroy'.

So when the next function needs to run on threads and re-initializes the structure, there will be conflicts whuch will cause hangs and crashes.

I am most greatful to Alan Lefor who patiently tested Gnuastro on his system many times until we were able to find the cause of the crash.