XML XInclude support

[Daniel Jacobowitz]

I wanted to index this as:

 @subsection Inclusion
+@cindex target descriptions, inclusion
+@cindex XInclude
+@cindex <xi:include>

That doesn't work in info, because the info browser tries to interpret
the colon.  But the colon is a syntactic element here, and this is what
I would look for.  Plus it works in the PDF, or in print.  What do you
think - should I leave the somewhat broken @cindex, since it's more
useful than not?

-- 
Daniel Jacobowitz
CodeSourcery