Homebrew isn't really designed to provide the keg only things to external build processes. They usually have those for other Homebrew packages which would build on top. I think their response would be "run `brew link readline` before you build". This doesn't help us very much, because automating this not easy nor ideal.

There doesn't exist a good solution as I see it currently. The only solution I could see would be to add some meta information to the rosdep rules, for example:

```yaml
libreadline:
  osx:
    homebrew:
      packages: [readline]
      link_on_install: true
```

Then the Homebrew installer for rosdep could run the link command after installation.