Use the real glibc openpty if present.

diff --git a/include/config.h.in b/include/config.h.in
index d154872..056dff3 100644
--- a/include/config.h.in
+++ b/include/config.h.in
@@ -182,6 +182,9 @@
 /* Define if you have the memmove function.  */
 #undef HAVE_MEMMOVE
 
+/* Define if you have the openpty function.  */
+#undef HAVE_OPENPTY
+
 /* Define if you have the rfork function.  */
 #undef HAVE_RFORK
 
@@ -305,6 +308,9 @@
 /* Define if you have the <netinet/tcp.h> header file.  */
 #undef HAVE_NETINET_TCP_H
 
+/* Define if you have the <pty.h> header file.  */
+#undef HAVE_PTY_H
+
 /* Define if you have the <resolv.h> header file.  */
 #undef HAVE_RESOLV_H