Codechange: Replace custom thread code with C++11 thread objects.

We assume a conforming C++11 compiler environment that has a valid <thread>-header.
Failure to run a real thread is handled gracefully.

src/linkgraph/linkgraphschedule.h

@@ -53,7 +53,7 @@ public:
 	static const uint SPAWN_JOIN_TICK = 21; ///< Tick when jobs are spawned or joined every day.
 	static LinkGraphSchedule instance;
 
-	static void Run(void *j);
+	static void Run(LinkGraphJob *job);
 	static void Clear();
 
 	void SpawnNext();