buildfix: check for old pod2man versions

Older pod2man don't have a --utf8 switch, check for this in conffigure
and use it only when present.  Fixes build on RHEL-5.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
