XML's XSD file is now searched :
1) As the file pointed by the environment variable VCSN_XSD_PATH,
2) In the current directory, as `vaucanson.xsd',
3) In the default installation path for this file (most likely
   `/usr/share/vaucanson/vaucanson.xsd').

- include/vaucanson/xml/xerces_parser.hh: Rename to...
- include/vaucanson/xml/xerces_parser-hh.in: ... this, and inherit
  `VCSN_XSD_PATH' from the configure.
- include/vaucanson/xml/xerces_parser.hxx: Check for the XSD file
  different locations.
- doc/xml/vaucanson.xsd: Move to...
- src/vaucanson.xsd: ... Here.
- doc/xml/Makefile.am: Update accordingly.
- src/Makefile.am: Distribute the XSD in a `data' directory (most
  likely `/usr/share').

- src/tests/xml/Makefile.am,
- src/demos/function_library/Makefile.am,
- src/demos/algorithms/Makefile.am,
- src/demos/transducers/Makefile.am: Update the path to the XSD in
  the symlink creation. Don't use GNU extensions of ln(1).

- configure.ac: Compute the default XSD path and substitute a variable
  in `xerces_parser-hh.in'.