configure.ac: Fix a2x check

configure.ac: Fix a2x check

* If enable_man_doc is set, but a2x can't be found, configure should
  fail instead of silently disabling man page creation.

* The AS_IF block checking $need_a2x is never active (need_a2x has been
  removed from configure.ac in 13e44a608 and a277479dc).

* AC_CHECK_PROG(VAR, ...) is a noop if VAR is already set, allowing the
  user to explicitly specify the (path to the) binary in VAR. Adjust the
  AS_IF check to account for this.

Signed-off-by: Luis Ressel <aranea@aixah.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>