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.
No worries at all! I wanted to push a more ambitious version of this PR to kick off pondering over which direction we could go in long-term, and understanding that this particular way is a little clunky is helpful feedback in my quest :wink:. 

I've been privately testing a whole bunch of OpenSSL/LibreSSL links over the past few weeks, as well as keeping track on the OpenBSD progress and Gentoo's push as well. Most things work, some things horribly break, but it's an exciting time, to have a security landscape that is expanding rapidly for the first time in a *loooooong* time.

I'll revert this back to the much simpler version, and I'll start pondering a requirement for when/if this option becomes adopted a little more. Short-term though, I'll try to reign in my ambitions and try to not make everyone profoundly disturbed by my enjoyment of this kind of niche interest :grinning: