curl: libressl support

New ground has been broken. New seeds have been sowed, etc etc. This makes the necessary changes to LibreSSL and cURL to enable the latter to be built with the former. 

This has been supported upstream for the last 3 months, and cURL officially supports LibreSSL [now](http://daniel.haxx.se/blog/2014/08/05/libressl-vs-boringssl-for-curl/) and [has done](http://curl.haxx.se/changes.html) since 7.38.0. 

A recompile of LibreSSL and cURL is necessary if you wish to adopt this locally, but the dependents of cURL should not need to be recompiled.
We're shying away from using `build.with?` whenever possible in the class body (e.g. outside `install`) so I'm going to ask if you can change this back to how it was before unfortunately (:sob:) and just add a `ohai` at the beginning of `install` letting people know it's using `openssl` if you pass both instead. The ideal solution is probably a requirement but I think it's overkill for this case.