Revision 1888617

Make task runner build & work without APR threading support.

* subversion/libsvn_subr/task.c
  (next_task, 
   worker_cancelled,
   send_terminate,
   worker,
   worker_thread,
   wait_for_outputting_state): Don't compile without APR threading.
  (execute_concurrently): Return a malfunction error when called
                          without APR threading.