Fix: Mark ASCIIDOC_ATTRS_CONF as a dependency of man page targets

ASCIIDOC_ATTRS_CONF contains the various paths set by autoconf,
such as datadir, syscondif and prefix, and it may be changed
by the user by invoking ./configure with different options. In
such a case, the man pages should be regenerated to take the new
paths into account.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>