Log Message:
Don't pretend to automatically add ${PREFIX} to each XML_ENTRIES and
SGML_ENTRIES entry - it doesn't work.  Probably because expansion
in this context requires the variable to already be defined, which it
isn't.

This is lucky, because every package using catalogs.mk adds ${PREFIX}
itself to each entry.  So usually it gets added just the once.

The luck stops if you do something like: make PREFIX=/usr/pkgroot2

This makes the automatic ${PREFIX} insertion suddenly work AS WELL as
the client packages adding ${PREFIX} themselves.  We end up with
/usr/pkgroot2/share/xml/catalog containing lines like:

<nextCatalog \ 
catalog="/usr/pkgroot2/usr/pkgroot2/share/xml/docbook/4.3/catalog" \ 
/>

... after installing textproc/docbook-xml.  This in turn causes
textproc/scrollkeeper to obscurely fail to build at configure time.