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/linkgraphjob.h

@@ -12,7 +12,7 @@
 #ifndef LINKGRAPHJOB_H
 #define LINKGRAPHJOB_H
 
-#include "../thread/thread.h"
+#include "../thread.h"
 #include "linkgraph.h"
 #include <list>
 
@@ -59,7 +59,7 @@ private:
 protected:
 	const LinkGraph link_graph;       ///< Link graph to by analyzed. Is copied when job is started and mustn't be modified later.
 	const LinkGraphSettings settings; ///< Copy of _settings_game.linkgraph at spawn time.
-	ThreadObject *thread;             ///< Thread the job is running in or NULL if it's running in the main thread.
+	std::thread thread;               ///< Thread the job is running in or a default-constructed thread if it's running in the main thread.
 	Date join_date;                   ///< Date when the job is to be joined.
 	NodeAnnotationVector nodes;       ///< Extra node data necessary for link graph calculation.
 	EdgeAnnotationMatrix edges;       ///< Extra edge data necessary for link graph calculation.
@@ -266,7 +266,7 @@ public:
 	 * Bare constructor, only for save/load. link_graph, join_date and actually
 	 * settings have to be brutally const-casted in order to populate them.
 	 */
-	LinkGraphJob() : settings(_settings_game.linkgraph), thread(NULL),
+	LinkGraphJob() : settings(_settings_game.linkgraph),
 			join_date(INVALID_DATE) {}
 
 	LinkGraphJob(const LinkGraph &orig);