conditional breakpoint:

break _CORE_message_queue_Seize
condition 1 the_message_queue->number_of_pending_messages == 64

conditional watchpoint:

watch ((CORE_message_queue_Control *) 0x408da6bc)->Pending_messages.last if \
  ((CORE_message_queue_Control *) 0x408da6bc)->Pending_messages.last == 0