BUILD/SMALL: threads: enable threads on osx

Enable multi thread on Darwin platform too.
diff --git a/Makefile b/Makefile
index 53a80d4..9cacb93 100644
--- a/Makefile
+++ b/Makefile
@@ -356,7 +356,7 @@
 # Mac OS/X
 ifeq ($(TARGET),osx)
   set_target_defaults = $(call default_opts, \
-    USE_POLL USE_TPROXY USE_LIBCRYPT USE_KQUEUE)
+    USE_POLL USE_TPROXY USE_LIBCRYPT USE_THREAD USE_KQUEUE)
   EXPORT_SYMBOL  = -export_dynamic
 endif