allow explicitly passing empty catalog argument to xmlcatalog
version: 1.33.2
when configuring, if --with-xml-catalog
is empty then the script will attempt to find the catalog file itself. This however breaks when the catalog files are not stored in /etc/xml. xmlcatalog has an escape hatch to deal with this by allowing the user/builder to set the $XML_CATALOG_FILES env var which is a space separated list of catalog files to search for entities.
ref: https://linux.die.net/man/1/xmlcatalog
NixOS carries the following patch https://github.com/NixOS/nixpkgs/blob/master/pkgs/development/tools/documentation/gtk-doc/respect-xml-catalog-files-var.patch to support this use case.