mod_netconf: set tcp port of notification server

add configure option to set tcp port of websockets server
- refs #643
diff --git a/src/configure.in b/src/configure.in
index 6b91bf3..4e56d69 100644
--- a/src/configure.in
+++ b/src/configure.in
@@ -72,6 +72,12 @@
 	]
 )
 
+AC_ARG_WITH([notifport],
+	[AC_HELP_STRING([--with-notifport=PORT], [Set TCP port of websocket notification server.])],
+	[CFLAGS="$CFLAGS -DNOTIFICATION_SERVER_PORT=$with_notifport"],
+	[CFLAGS="$CFLAGS"]
+)
+
 CFLAGS="$CFLAGS -I/usr/include/json"
 AC_SUBST(REQS)
 AC_SUBST(BUILDREQS)