Use a boolean instead as thread's state

Now that we have just two mutually exclusive thread's states
we can repleace them by a simple boolean.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/thread.h

@@ -65,12 +65,6 @@ struct SplitPoint {
 
 struct Thread {
 
-  enum ThreadState
-  {
-    AVAILABLE,     // Thread is waiting for work
-    SEARCHING      // Thread is performing work
-  };
-
   void wake_up();
   bool cutoff_occurred() const;
   bool is_available_to(int master) const;
@@ -83,9 +77,9 @@ struct Thread {
   int maxPly;
   Lock sleepLock;
   WaitCondition sleepCond;
-  volatile ThreadState state;
   SplitPoint* volatile splitPoint;
   volatile int activeSplitPoints;
+  volatile bool is_searching;
   volatile bool do_sleep;
   volatile bool do_terminate;