Mac build fix. Macs don't support timeouts on locks.

Change-Id: I75665300d9b99420972865109026c8c090748cef
diff --git a/src/mutex.cc b/src/mutex.cc
index dc33fbe..18c9453 100644
--- a/src/mutex.cc
+++ b/src/mutex.cc
@@ -302,10 +302,8 @@
   CHECK_MUTEX_CALL(pthread_rwlock_unlock, (&rwlock_));
 }
 
+#if HAVE_TIMED_RWLOCK
 bool ReaderWriterMutex::ExclusiveLockWithTimeout(const timespec& abs_timeout) {
-  // Check that timeouts are supported. Currently Darwin doesn't support timeouts, if we are
-  // running on Darwin the timeout won't be respected.
-#if defined(_POSIX_TIMEOUTS) && (_POSIX_TIMEOUTS - 200112) >= 0L
   int result = pthread_rwlock_timedwrlock(&rwlock_, &abs_timeout);
   if (result == ETIMEDOUT) {
     return false;
@@ -317,12 +315,8 @@
   RegisterAsLockedWithCurrentThread();
   AssertSharedHeld();
   return true;
-#else
-  UNUSED(abs_timeout);
-  ExclusiveLock();
-  return true;
-#endif
 }
+#endif
 
 void ReaderWriterMutex::SharedLock() {
   CHECK_MUTEX_CALL(pthread_rwlock_rdlock, (&rwlock_));