Previously, we created a variant of the installation instructions for
producing the plain-text INSTALL file by marking up certain parts of
installation.sgml using SGML parameterized marked sections.  Marked
sections will not work anymore in XML, so before we can convert the
documentation to XML, we need a new approach.

DocBook provides a "profiling" feature that allows selecting content
based on attributes, which would work here.  But it imposes a noticeable
overhead when building the full documentation and causes complications
when building some output formats, and given that we recently spent a
fair amount of effort optimizing the documentation build time, it seems
sad to have to accept that.

So as an alternative, (1) we create our own mini-profiling layer that
adjusts just the text we want, and (2) assemble the pieces of content
that we want in the INSTALL file using XInclude.  That way, there is no
overhead when building the full documentation and most of the "ugly"
stuff in installation.sgml can be removed and dealt with out of line.