Woops, I missed that.  It's very useful if this could pass, or else it'll constantly flag up as a failure in our automated bulk runs.  One option is to add a source build tag, which runs after depexts and can run an arbitrary script to do a source (or PPA) installation.

If it's easy to install zmq4 from source, you just need something like this in the depexts (this example taken from the `sodium` package):

```
depexts: [
  [ ["source" "linux"] ["https://gist.github.com/avsm/9037146/raw"] ]
]
```