Fix C++11 Windows build of threading code

	The initial problem was that mutex_auto_lock.h tries to use std::unique_lock<std::mutex>
	despite mutex.h not using C++11's std::mutex on Windows. The problem here is the mismatch
	between C++11 usage conditions of the two headers. This commit moves the decision logic
	to threads.h and makes sure mutex.h, mutex_auto_lock.h and event.h all use the same features.