shout2: Add compatibility for libshout >= 2.4.2

In libshout >=2.4.2 shout_open() can return SHOUTERR_RETRY in addition
to SHOUTERR_BUSY.

The nonblocking example in libshout fixes the problem in a similar
way, as mentioned by the author in this issue:
https://gitlab.xiph.org/xiph/icecast-libshout/-/issues/2316

Fixes gstreamer/gst-plugins-good#848

Part-of: <gstreamer/gst-plugins-good!1108>

ext/shout2/gstshout2.c

@@ -567,7 +567,13 @@ gst_shout2send_connect (GstShout2send * sink)
   ret = shout_open (sink->conn);
 
   /* wait for connection or timeout */
+#ifdef SHOUTERR_RETRY
+  /* starting with libshout 2.4.2, shout_open() has broken API + ABI and
+   * can also return SHOUTERR_RETRY (a new define) to mean "try again" */
+  while (ret == SHOUTERR_BUSY || ret == SHOUTERR_RETRY) {
+#else
   while (ret == SHOUTERR_BUSY) {
+#endif
     if (gst_util_get_timestamp () - start_ts > sink->timeout * GST_MSECOND) {
       goto connection_timeout;
     }