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/event.h

@@ -26,17 +26,12 @@ DEALINGS IN THE SOFTWARE.
 #ifndef THREADING_EVENT_H
 #define THREADING_EVENT_H
 
-#if __cplusplus >= 201103L
+#include "threads.h"
+
+#if USE_CPP11_MUTEX
 	#include <condition_variable>
 	#include "threading/mutex.h"
 	#include "threading/mutex_auto_lock.h"
-#elif defined(_WIN32)
-	#ifndef WIN32_LEAN_AND_MEAN
-		#define WIN32_LEAN_AND_MEAN
-	#endif
-	#include <windows.h>
-#else
-	#include <pthread.h>
 #endif
 
 
@@ -49,18 +44,18 @@ DEALINGS IN THE SOFTWARE.
 class Event {
 public:
 	Event();
-#if __cplusplus < 201103L
+#ifndef USE_CPP11_MUTEX
 	~Event();
 #endif
 	void wait();
 	void signal();
 
 private:
-#if __cplusplus >= 201103L
+#if USE_CPP11_MUTEX
 	std::condition_variable cv;
 	Mutex mutex;
 	bool notified;
-#elif defined(_WIN32)
+#elif USE_WIN_MUTEX
 	HANDLE event;
 #else
 	pthread_cond_t cv;