We actually do still need config.h to get the 'const' definition for K&R C.

strerror.c

@@ -35,6 +35,8 @@
 #include <stdio.h>
 #include <errno.h>
 
+#include "config.h"
+
 #ifndef lint
 static const char rcsid[] = "$Sudo$";
 #endif /* lint */