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.h

@@ -12,7 +12,7 @@
 #ifndef PROGRESS_H
 #define PROGRESS_H
 
-#include "thread/thread.h"
+#include <mutex>
 
 static const uint MODAL_PROGRESS_REDRAW_TIMEOUT = 200; ///< Timeout between redraws
 
@@ -26,10 +26,20 @@ static inline bool HasModalProgress()
 	return _in_modal_progress;
 }
 
+/**
+ * Check if we can use a thread for modal progress.
+ * @return Threading usable?
+ */
+static inline bool UseThreadedModelProgress()
+{
+	extern bool _use_threaded_modal_progress;
+	return _use_threaded_modal_progress;
+}
+
 bool IsFirstModalProgressLoop();
 void SetModalProgress(bool state);
 
-extern class ThreadMutex *_modal_progress_work_mutex;
-extern class ThreadMutex *_modal_progress_paint_mutex;
+extern std::mutex _modal_progress_work_mutex;
+extern std::mutex _modal_progress_paint_mutex;
 
 #endif /* PROGRESS_H */