configure: Properly support disabling AMQP

Tie --disable-amqp and --without-librabbitmq-client together: if
either is set, disable the other too, and in both cases, clear
LIBRABBITMQ_SUBDIRS.

This means that disabling AMQP will neither build the module, nor will
it recurse into modules/afamqp/librabbitmq-client.

Reported-by: Jose Pedro Oliveira <jpo@di.uminho.pt>
Signed-off-by: Gergely Nagy <algernon@balabit.hu>