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.
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