Enable multithreading.
diff --git a/Mac/mwerks/mwerks_plugin_config.h b/Mac/mwerks/mwerks_plugin_config.h
index 1bb465d..1630c6c 100644
--- a/Mac/mwerks/mwerks_plugin_config.h
+++ b/Mac/mwerks/mwerks_plugin_config.h
@@ -4,6 +4,7 @@
#define USE_GUSI1 /* Stdio implemented with GUSI */
/* #define USE_GUSI2 /* Stdio implemented with GUSI */
+#define WITH_THREAD /* Use thread support (needs GUSI 2, not GUSI 1) */
#define USE_MSL /* Use MSL libraries */
#ifdef USE_MSL
#define MSL_USE_PRECOMPILED_HEADERS 0 /* Don't use precomp headers: we include our own */