WIP: Add support to use system libraries

[guillemj]

These patches add support to optionally use system libraries instead of the vendored (or embedded) projects. This was requested in #493. I'm sending what I've got for now to avoid duplicated work, even though this does not seem to be passing the tests yet. I'll try to check which commit makes it fail next week. I've left out for now the lua vendored code, because I cannot test that with what is currently packaged in Debian as there are at least two lua modules missing there. My current WIP for that is in https://github.com/guillemj/KeyDB/commit/5bbff3c91e99f87a806888c6f8ddc1c308c6e158.

This is something that is required by most major distributions as a matter of policy, and something that will need to be carried over when packaging for Debian (or Fedora I guess), see https://bugs.debian.org/1067413.