--- a/include/sm/config.h
+++ b/include/sm/config.h
@@ -40,9 +40,7 @@
 **  Configuration macros that specify how __P is defined.
 */
 
-# ifndef SM_CONF_SYS_CDEFS_H
-#  define SM_CONF_SYS_CDEFS_H		0
-# endif /* ! SM_CONF_SYS_CDEFS_H */
+# undef SM_CONF_SYS_CDEFS_H
 
 /*
 **  SM_CONF_STDDEF_H is 1 if <stddef.h> exists