diff --git a/src/s/irix5-0.h b/src/s/irix5-0.h
index 9d3497c7cac..ea39a51eeec 100644
--- a/src/s/irix5-0.h
+++ b/src/s/irix5-0.h
@@ -110,3 +110,7 @@ char *_getpty();
 
 #define MAIL_USE_FLOCK
 
+/* use K&R C */
+#ifndef __GNUC__
+#define C_SWITCH_SYSTEM -cckr
+#endif