Use more compatible initializarion of RWLockPosix

drivers/unix/rw_lock_posix.cpp

@@ -63,7 +63,8 @@ void RWLockPosix::make_default() {
 
 RWLockPosix::RWLockPosix() {
 
-	rwlock=PTHREAD_RWLOCK_INITIALIZER;
+	//rwlock=PTHREAD_RWLOCK_INITIALIZER; fails on OSX
+	pthread_rwlock_init(&rwlock,NULL);
 }