- include/vaucanson/config/system.hh: Define VCSN_INCLUDE_XML(),
valuated to `true' if XML files can be included.

- include/vaucanson/algebra/implementation/series/rat/xml_exp_visitor.hh,
- include/vaucanson/algebra/implementation/series/rat/exp.hh,
- include/vaucanson/algebra/implementation/series/rat/exp.hxx,
- include/vaucanson/xml/xml_chooser.hh,
- include/vaucanson/xml/session.hh,
- include/vaucanson/xml/node.hh,
- include/vaucanson/xml/error_handler.hh,
- include/vaucanson/xml/xml_converter.hh,
- include/vaucanson/xml/strings.hh,
- include/vaucanson/xml/ios.hh,
- include/vaucanson/xml/XML.hh,
- include/vaucanson/xml/tools.hh,
- include/vaucanson/xml/xerces_parser.hh,
- include/vaucanson/tools/xml_dump.hh,
- include/vaucanson/tools/xml_display.hh,
- include/vaucanson/tools/xml_load.hh: Use it.

- build-aux/check_xml.sh: Say that VCSN_INCLUDE_XML() is a valid test
of XML include'ability.