Add a check for sys/modem.h, needed by termios on HP-UX.
diff --git a/config.h.in b/config.h.in
index 1e97f97..6753327 100644
--- a/config.h.in
+++ b/config.h.in
@@ -590,6 +590,9 @@
 /* Define if you have the <sys/lock.h> header file.  */
 #undef HAVE_SYS_LOCK_H
 
+/* Define if you have the <sys/modem.h> header file.  */
+#undef HAVE_SYS_MODEM_H
+
 /* Define if you have the <sys/ndir.h> header file.  */
 #undef HAVE_SYS_NDIR_H