Added a configure test for pthread_cond_clockwait presence. If it is
available, use it for timed waits with absolute timeouts on POSIX platforms.
Futex-based implementation has been improvved to use proper atomic operations
to access m_cond, which is needed to avoid races between the futex syscall and
our notifying operations. Also, check if m_cond has changed whil looping in
the waiting operations.
