gtk_doc: replace catalog lookup hack

In the previous commit, we added a setup hook to docbook dtd and xsl
packages, that adds derivation’s catalog file to an environment variable.
That should, in theory, remove the need for declaring their catalogs manually.
Unfortunately, xmlcatalog utility expects exactly one catalog file, completely
disregarding the environment variable in non-interactive context. In the same
spirit, the design of gtk-doc m4 files only admits a single catalog file,
resulting in another ugly hack.