gthread: Use pthread_cond_timedwait_monotonic() if available

gthread: Use pthread_cond_timedwait_monotonic() if available

Otherwise we have to rely on pthread_cond_timedwait() actually using
the monotonic clock, which might be true or not. On Android at least
it is using the realtime clock, no pthread_condattr_setclock() is available
but instead pthread_cond_timedwait_monotonic() can be used.