- check_xml.sh: Rewrite a bit. Check if the hh that includes the hxx or
hcc already checks for USE_XML. Same with the Makefile.am.
- include/Makefile.am: Install XML files iif VCSN_USE_XML is set.

- Makefile.am,
- include/vaucanson/tools/out_display.hh,
- include/vaucanson/design_pattern/factory.hh: Pacify `check_xml' by
talking about VCSN_USE_XML in a comment.

- src/tests/Makefile.am: Check for XML only if XML_CHECK.
- src/demos/automaton_library/Makefile.am,
- src/demos/transducers/one_rule_rewriting/Makefile.am,
- doc/xml/Makefile.am: Surround code by tests for VCSN_USE_XML or
XML_CHECK.