Put in an earlier include of sys/signal.h (within ifdef USE_GUSI) to forestal a multiple define.
diff --git a/Mac/tclmods/tclSelectNotify.c b/Mac/tclmods/tclSelectNotify.c
index ea35fc6..a728442 100644
--- a/Mac/tclmods/tclSelectNotify.c
+++ b/Mac/tclmods/tclSelectNotify.c
@@ -23,6 +23,10 @@
 	#pragma import reset
 #endif
 
+#ifdef USE_GUSI
+/* Move this include up otherwise tclPort.h tried to redefine signals */
+#include <sys/signal.h>
+#endif
 #include "tclInt.h"
 #include "tclPort.h"
 #include <signal.h>