configure.ac: Improve the --with-libmongo-client behaviour

Make --with-libmongo-client support an "auto" setting, and make that the
default. This fixes #83.

Reported-by: Andras Mitzki <micek@balabit.hu>
Signed-off-by: Gergely Nagy <algernon@balabit.hu>
Merge pull request #84 from algernon/h/with-libmongo-client-auto

configure.ac: Improve the --with-libmongo-client behaviour