implement barrier/critical section for OSX (PosixThreadSupport)

enable Demos/ThreadingDemo for OSX
add cmake build support for VectorAdd_OpenCL for OSX

src/BulletMultiThreaded/btThreadSupportInterface.h

@@ -76,6 +76,8 @@ public:
 	virtual btBarrier*	createBarrier() = 0;
 
 	virtual btCriticalSection* createCriticalSection() = 0;
+	
+	virtual void*	getThreadLocalMemory(int taskId) { return 0; }
 
 };