Codechange: Replace custom mutex code with C++11 mutex'es.

A conforming compiler with a valid <mutex>-header is expected.
Most parts of the code assume that locking a mutex will never fail unexpectedly,
which is generally true on all common platforms that don't just pretend to
be C++11. The use of condition variables in driver code is checked.

src/genworld_gui.cpp

@@ -1321,10 +1321,10 @@ static void _SetGeneratingWorldProgress(GenWorldProgress cls, uint progress, uin
 	 * paint thread. The 'other' thread already has the paint thread rights so
 	 * this ensures us that we are waiting until the paint thread is done
 	 * before we reacquire the mapgen rights */
-	_modal_progress_work_mutex->EndCritical();
-	_modal_progress_paint_mutex->BeginCritical();
-	_modal_progress_work_mutex->BeginCritical();
-	_modal_progress_paint_mutex->EndCritical();
+	_modal_progress_work_mutex.unlock();
+	_modal_progress_paint_mutex.lock();
+	_modal_progress_work_mutex.lock();
+	_modal_progress_paint_mutex.unlock();
 
 	_gws.timer = _realtime_tick;
 }