cmake: Find docbook-xsl using XML catalog
Looking at random paths in CMAKE_INSTALL_DATAROOTDIR will only work when the XSL stylesheets are installed to the same prefix as dbus, which is not the case on Nix.
Let’s use libxml’s xmlcatalog program that will try to locate the stylesheet in system’s global catalog (/etc/xml/catalog) or one of the catalogs listed in XML_CATALOG_FILES environment variable, when set.