Work around the lack of gettid(3) on Mac OS.

This just disables the checks that need a tid.

Change-Id: I5fb39db9e36f896cbfa55dffb974677dc0f033e2
diff --git a/src/mutex.cc b/src/mutex.cc
index d41c6cb..fca31b6 100644
--- a/src/mutex.cc
+++ b/src/mutex.cc
@@ -76,6 +76,9 @@
     // ...other stuff we don't care about.
   };
   return reinterpret_cast<glibc_pthread_t*>(&mutex_)->owner;
+#elif defined(__APPLE__)
+  // We don't know a way to implement this for Mac OS.
+  return 0;
 #else
   UNIMPLEMENTED(FATAL);
   return 0;
@@ -93,6 +96,8 @@
     // ...other stuff we don't care about.
   };
   reinterpret_cast<glibc_pthread_t*>(&mutex_)->owner = 0;
+#elif defined(__APPLE__)
+  // We don't know a way to implement this for Mac OS.
 #else
   UNIMPLEMENTED(FATAL);
 #endif