configure: support for the "docbook2man" utility to build the documentation

This adds docbook2man as an alternative name for the docbook compiler.
As that name was used on Debian based systems for an older version of the tool,
this change also adds a check so that docbook2man is never used on Debian based
systems.

Reported-by: Peter Simons <simons@cryp.to>
Reported-by: Christian Bühler christian@cbuehler.de
Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>