* libxml2 / libxslt: provide a setup hook that prevents xmllint and

  xsltproc from downloading DTDs from the network, which is impure.
  This caused a lot of problems with Gnome builds in the build farm,
  because those downloads are slow and would sometimes fail.

  The setup hook also sets up $XML_CATALOG_FILES.

svn path=/nixpkgs/trunk/; revision=9471