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.

src/threading/mutex_auto_lock.h

@@ -26,7 +26,9 @@ DEALINGS IN THE SOFTWARE.
 #ifndef THREADING_MUTEX_AUTO_LOCK_H
 #define THREADING_MUTEX_AUTO_LOCK_H
 
-#if __cplusplus >= 201103L
+#include "threads.h"
+
+#if USE_CPP11_MUTEX
 	#include <mutex>
 	using MutexAutoLock = std::unique_lock<std::mutex>;
 	using RecursiveMutexAutoLock = std::unique_lock<std::recursive_mutex>;