$NetBSD$

On NetBSD, expose ptsname_r prototype from stdlib.h

--- configure.ac.orig	2016-11-09 20:40:33.000000000 +0100
+++ configure.ac	2018-02-11 09:24:29.454059400 +0100
@@ -57,6 +57,7 @@
              #ifndef _GNU_SOURCE
              #define _GNU_SOURCE
              #endif
+             #define _NETBSD_SOURCE
              #include <stdlib.h>],
             [char buf[10]; ptsname_r(0, buf, sizeof(buf));],
             [AC_DEFINE(HAVE_PTSNAME_R, 1,