[PATCH] Add build info tag to squid -v output

Hi all,
  a followup to yesterday's and today's IRC discussion.
This patch adds a configure option --enable-build-info[=text]

If text is specified, it will print it on the second line of "squid -v".
If the option is given but no text is specified, configure will
attempt to determine whether we are in a bzr checkout and locate the
bzr executable. If both succeed, it will automatically build a text
string containing the branch nickname and revno.
If no option is given, nothing will happen.

This patch serves two purposes:
- help those who closely trail trunk or some other bzr branch to
easily pinpoint good revisions
- help those who develop custom builds (e.g. for a specific customer)
explicitly mark it as such.

-- 
    /kinkie