add support for ZeroMQ

ZeroMQ can be used in FFmpeg to dynamically control filters while FFmpeg is running, by sending ZeroMQ messages to FFmpeg.

This is the only (good) way for scripts to interactively vary a filter values in real time, for example to change the volume applied to a source in real time in response to some user input.

ZeroMQ support is built in to the Debian 1 and Fedora 2 packages at the time of writing, is quite small, and fits an important need, so I believe it makes sense to add this to the Archlinux build as well.