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 */
diff --git a/Mac/mwerks/mwerks_shared_config.h b/Mac/mwerks/mwerks_shared_config.h
index 66174b5..becd670 100644
--- a/Mac/mwerks/mwerks_shared_config.h
+++ b/Mac/mwerks/mwerks_shared_config.h
@@ -8,6 +8,7 @@
/* #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 Mw Standard Library (as opposed to Plaugher C libraries) */
#define USE_CORE_TOOLBOX /* Include core toolbox modules (Dlg,Ctl,Menu,Win,Res,Qd) */
/* #define USE_TOOLBOX /* Include all toolbox modules in core Python */
diff --git a/Mac/mwerks/mwerks_tkplugin_config.h b/Mac/mwerks/mwerks_tkplugin_config.h
index 88b7552..bd3b474 100644
--- a/Mac/mwerks/mwerks_tkplugin_config.h
+++ b/Mac/mwerks/mwerks_tkplugin_config.h
@@ -4,5 +4,6 @@
#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_TK /* Include _tkinter module in core Python */
#define MAC_TCL /* This *must* be on if USE_TK is on */