The --stringparam option is not available on older xmlto versions.
Instead, set man.base.url.for.relative.links via a .xsl file. Older
docbook versions will ignore this without causing grief to users of
older xmlto versions.
Signed-off-by: Todd Zullinger <tmz@pobox.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>