package/syslog-ng: add optional rabbitmq-c dependency

rabbitmq-c is not an embedded submodule since version 3.16.1 and
https://github.com/balabit/syslog-ng/commit/c0559593c377f04662368dbecf282d2670aad12f



So enable/disable amqp depending on rabbitmq-c availability and remove
uneeded -lrt from LIBS

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Reviewed-by: Chris Packham <judge.packham@gmail.com>
Signed-off-by: Fabrice Fontaine &lt;<a href="mailto:fontaine.fabrice@gmail.com" target="_blank" rel="noreferrer">fontaine.fabrice@gmail.com</a>&gt;<br></blockquote></div></div><div dir="auto"><br></div><div dir="auto">Looks good to me</div><div dir="auto"><br></div><div dir="auto">Reviewed-by: Chris Packham &lt;<a href="mailto:judge.packham@gmail.com">judge.packham@gmail.com</a>&gt;<br><br></div><div dir="auto"><div class="gmail_quote"><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>