Add a STYLESHEET_TYPE variable, which defaults to 'dsssl'. When set to
'xsl' a different set of rules are invoked to use an XSL toolchain
(processors, stylesheets, and so forth) to convert the DocBook to the
various different output formats.

I haven't actually written the rules that are invoked when this knob is
set to 'xsl'. But how hard can it be. . .