See if multithreaded dex2oat is causing the Mac OS 10.6 build problems.

(Everything's working fine for me on 10.7.)

Change-Id: I1e3cf3dd818ca00f986fb0e31f4f4a3a69df5daa
diff --git a/src/dex2oat.cc b/src/dex2oat.cc
index 660115c..fbe55a3 100644
--- a/src/dex2oat.cc
+++ b/src/dex2oat.cc
@@ -554,6 +554,10 @@
     }
   }
 
+#if defined(__APPLE__)
+  thread_count = 1;
+#endif
+
   if (oat_filename.empty() && oat_fd == -1) {
     Usage("Output must be supplied with either --oat-file or --oat-fd");
   }