org-bbdb: Check for BBDB version depending on available functions
* org-bbdb.el (org-bbdb-old): Replace `defvar' after loading 'bbdb to
check for old or new version by a check for the required function in
`org-bbdb-open' and `org-bbdb-store-link'.
(org-bbdb-store-link, org-bbdb-open): Check which version of bbdb is
to be used.
This is a follow up of
ddf4f1e7a95608b2223dee7170ecd4a51788def4.
Instead of depending on the loading of bbdb we do a check for the
required function in `org-bbdb-open' and `org-bbdb-store-link'.