[cedet-semantic] Auto-create GNU GLOBAL database hook before using Semantic Symref

[Per N. <per...@gm...>]

If we choose the GNU GLOBAL backend for Symref, I believe it should
automatically on-demand create (gtags) and update (global -u) a GNU GLOBAL
database in current project root. We could use advices for that but it would
be cleaner if we add a hook, say (defun symref-pre-hook (backend
directory)), in some suitable place.

/Per