Since asciidoc is based on Phyton 2, we want to move to alternative,
and a promising one is asciidoctor.  This patch allows to use
asciidoctor for formatting man pages instead of asiidoc, by passing
a make option, USE_ASCIIDOCTOR=yes.

Although asciidoctor is almost compatible with asciidoc, it can
produce a man page directly from a text file without XML, which is
more efficiently.  So in asciidoctor mode, we don't require xmlto.
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Andrei Vagin <avagin@gmail.com>