dirmngr: Allow building with libassuan < 2.3.

* dirmngr/dirmngr.c (set_tor_mode): Use newer assuan function only if
available.
* dirmngr/http.c (http_raw_connect): Ditto.
--

Frankly we should require that but we can also wait for 2.4.0 and
switch then.

dirmngr/dirmngr.c

@@ -470,7 +470,9 @@ set_tor_mode (void)
 {
   if (opt.use_tor)
     {
+#if ASSUAN_VERSION_NUMBER >= 0x020300 /* >= 2.3.0 */
       if (assuan_sock_set_flag (ASSUAN_INVALID_FD, "tor-mode", 1))
+#endif
         {
           log_error ("error enabling TOR mode: %s\n", strerror (errno));
           log_info ("(is your Libassuan recent enough?)\n");