Give a better error message when docbook5 files are missing

[strk]

Rather than these scary messages: ​https://github.com/postgis/postgis/actions/runs/6165616469/job/16733693326

It would be useful to know, say, at ./configure time, that the required docbook 5 tools are missing.

We need docbook5-xml to validate the XML and possibly also to create the html and pdf and epub versions

A possible way to check could be running:

xmlcatalog "" '​http://docbook.org/xml/5.0/rng/docbook.rng'

Which should exit with an error when docbook5-xml package is missing, although we might be more specific about what we need

[Sandro Santilli <strk@…>]

In d9c9781/git:

Fail early and avoid error flood if docbook5 isn't found

References #5533

[strk]

I think this is good enough as an error: ​https://github.com/postgis/postgis/actions/runs/6169683106/job/16744432185#step:4:2032

Closing as fixed