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 */