Documentation: xmlto 0.0.18 does not know --stringparam

Documentation: xmlto 0.0.18 does not know --stringparam

Newer DocBook stylesheets want man.base.url.for.relative.links
parameter set when formatting manpages with external references
to turn them into full URLs, and leave a helpful "you should
set this parameter" message in the output.  Earlier we added
the MAN_BASE_URL make variable to specify the value for it.

When MAN_BASE_URL is not given, it ought to be safe to set the
parameter to empty; it would result in an empty leading path for
older stylesheets that ignore the parameter, and newer ones
would produce the same "relative URL" without the message.

Unfortunately, older xmlto (at least version 0.0.18 released in
early 2004 that comes with RHEL/CentOS 5) does not understand
the --stringparam command line option, so we cannot add the
parameter definition unconditionally to the command line.  Work
it around by passing the parameter only when set.

If you do not have a suitable URL prefix, you can pass a quoted empty
string to it, like so:

    $ make MAN_BASE_URL='""'

Signed-off-by: Junio C Hamano <gitster@pobox.com>