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/progress.cpp

@@ -10,17 +10,19 @@
 /** @file progress.cpp Functions for modal progress windows. */
 
 #include "stdafx.h"
-#include "thread/thread.h"
+#include "progress.h"
 
 #include "safeguards.h"
 
 /** Are we in a modal progress or not? */
 bool _in_modal_progress = false;
 bool _first_in_modal_loop = false;
+/** Threading usable for modal progress? */
+bool _use_threaded_modal_progress = true;
 /** Rights for the performing work. */
-ThreadMutex *_modal_progress_work_mutex = ThreadMutex::New();
+std::mutex _modal_progress_work_mutex;
 /** Rights for the painting. */
-ThreadMutex *_modal_progress_paint_mutex = ThreadMutex::New();
+std::mutex _modal_progress_paint_mutex;
 
 /**
  * Set the modal progress state.