zeromq: depend on libsodium if available

ZeroMQ doesn't require libsodium, but will enable CURVE security if it's
available. This patch adds the optional dependency.

[Thomas: pass --without-libsodium explicitly when libsodium is not
enabled, and to achieve this, backport a patch from upstream zeromq.]

Signed-off-by: Frank Hunleth <fhunleth@troodon-software.com>
Reviewed-by: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>