Here are some references to help you understand what "keg-only" means in the context of Homebrew:

https://github.com/mxcl/homebrew/wiki/FAQ#what-does-keg-only-mean
http://stackoverflow.com/questions/17015285/understand-homebrew-and-keg-only-dependencies
http://stackoverflow.com/questions/4691403/keg-only-homebrew-formulas

Do you use Macports exclusively or have you tried this with Homebrew? From what I can tell unless you manually did `brew link readline` after installing it, building your package will use the OS X provided version of readline and not the Homebrew one.

P.S.: consider using github's webui to reply, it doesn't handle your email clients quoting very well.