* Tell gtk-doc where the XML catalog is
The gtk-doc configure script has an option for specifying the path to
the XML catalog. If this is not set the configure script will search
a defined set of directories for a catalog file and will set
`with_xml_catalog` based on that. Only if no system catalog is found will
the XML_CATALOG_FILES be looked at. In order to make sure that the spack
provided catalog is used, pass the `--with-xml-catalog` option.
* Use the property from docbook-xml
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