Patch for porting-howto.xml

[Phil Edwards]

> > Also, how do I regenerate porting-howto.html?
> 
> Dunno.

The question has never come up before.  Usually Felix sends me a patch to
both the XML and HTML versions, and then I check them in with a message like

    2001-11-05  Felix Natter  <fnatter@gmx.net>

        * docs/html/17_intro/porting-howto.xml:  check in v0.9.4
        * docs/html/17_intro/porting-howto.html:  regenerate


>From the generated HTML, it looks like some DocBook tool is being used.
(Felix?  Help us out here?)


I suggest adding a line or two to docs/html/Makefile, which exists for
just such occasions.


Phil