Log Message:
Upgrade libxml2 to 2.6.7.

2.6.7: Feb 23 2004:
   - documentation: tutorial updates (John Fleck), benchmark results
   - xmlWriter: updates and fixes (Alfred Mickautsch, Lucas Brasilino)
   - XPath optimization (Petr Pajas)
   - DTD ID handling optimization
   - bugfixes: xpath number with  > 19 fractional (William Brack), push
    mode with unescaped '>' characters, fix xmllint --stream --timing, fix
    xmllint --memory --stream memory usage, xmlAttrSerializeTxtContent
    handling NULL, trying to fix Relax-NG/Perl interface.
   - python: 2.3 compatibility, whitespace fixes (Malcolm Tredinnick)
   - Added relaxng option to xmllint --shell