xmpp: add Common Lisp support library & integrate with cmake

- Like in danger-bird2, some Common Lisp gets compiled at weechat build
  time into a static library, which gets linked in to the final binary.
- This required some cmake trickery to get it to actually do the thing.
- As a proof of concept, we now get the "plugin loaded" message to be
  printed from Lisp, with the ECL version chucked in as a nifty trick.