BUILD: enable USE_THREAD for OpenBSD build.
diff --git a/Makefile b/Makefile
index f066f31..c897c02 100644
--- a/Makefile
+++ b/Makefile
@@ -335,6 +335,7 @@
   USE_KQUEUE     = implicit
   USE_TPROXY     = implicit
   USE_ACCEPT4    = implicit
+  USE_THREAD     = implicit
 else
 ifeq ($(TARGET),netbsd)
   # This is for NetBSD