npc(1): without <sysexits.h> fall back to ANSI C

src/npc.c

@@ -18,9 +18,13 @@
 
 #include <stdio.h> /* fprintf(3), fputs(3), getc(3), putc(3), stdin, stdout,
                     * EOF */
+#include <stdlib.h> /* EXIT_FAILURE, EXIT_SUCCESS */
 #include <unistd.h> /* getopt(3) */
-#if !defined EX_USAGE || !defined EX_OK
-#	include <sysexits.h>
+#if !defined EX_USAGE
+#	define EX_USAGE EXIT_FAILURE
+#endif
+#if !defined EX_OK
+#	define EX_OK EXIT_SUCCESS
 #endif
 
 int main(int argc, char *argv[]){