See https://bugs.freedesktop.org/show_bug.cgi?id=47792;
should become obsolete with the next release.

diff -u luit-1.1.1.orig/sys.c luit-1.1.1/sys.c
--- luit-1.1.1.orig/sys.c	2012-03-23 05:23:07.000000000 +0100
+++ luit-1.1.1/sys.c	2013-11-27 22:45:53.000000000 +0100
@@ -20,6 +20,8 @@
 THE SOFTWARE.
 */
 
+#define _XOPEN_SOURCE 600
+
 #ifdef HAVE_CONFIG_H
 # include "config.h"
 #endif
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<tr><td><span title='2017-12-23 00:05:05 +0100'>2017-12-23</span></td><td><a href='/guix/commit/tests/scripts-build.scm?id=3e30cdf1c35ebeb52630ec19b3b43b9e6d5ffb81'>guix build: Support '--with-source=PACKAGE@VERSION=URI'.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
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<tr><td><span title='2016-10-17 23:59:02 +0200'>2016-10-17</span></td><td><a href='/guix/commit/tests/scripts-build.scm?id=645b9df858683dc05ffa04c9eb2fdc45ccef4a65'>guix build: Add '--with-graft'.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>