Cl-lib recipe and improved :builtin property.

`:builtin` currently only matches against the major version number.  Cl-lib however is builtin since version 24.3.  To allow finer matching of version numbers I changed `el-get-package-method` to accept a string for `:builtin` and match it against `emacs-version` using `version<=`.

I also added a recipe for cl-lib and updated the one for weechat.el to use it.

Please **don't** merge this request, yet!  I haven't updated the documentation.  I want to know if my change to `:builtin` is acceptable first.