Restrict Mac OS 10.5 devices to 1 dex2oat thread.

Also make extra sure we've included the header file that tells us what Mac OS
version we're compiling for.

Change-Id: Ie57f01a325217e638df4e9b90d94b27303149124
diff --git a/src/mutex.cc b/src/mutex.cc
index da1ba7a..b56f1ef 100644
--- a/src/mutex.cc
+++ b/src/mutex.cc
@@ -23,6 +23,10 @@
 #include "thread.h"
 #include "utils.h"
 
+#if defined(__APPLE__)
+#include "AvailabilityMacros.h" // For MAC_OS_X_VERSION_MAX_ALLOWED
+#endif
+
 #define CHECK_MUTEX_CALL(call, args) CHECK_PTHREAD_CALL(call, args, name_)
 
 namespace art {