https://bugs.gentoo.org/880807
--- a/src/fixio.c
+++ b/src/fixio.c
@@ -10,9 +10,11 @@
  * $Id: fixio.c,v 1.4 2002/01/28 01:16:52 jason Exp $
  */
 
+#define _XOPEN_SOURCE 600
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <unistd.h>
 #include "shorten.h"
 #include "bitshift.h"