Log Message:
Update to 1.68:

1.68pre
     - fix compilation problems
     - skip exslt tests if not available
     - added XML::LibXSLT::HAVE_EXSLT() (1/0)

1.67
     - sync the binary internals to those of XML::LibXML 1.67
     - update regression tests for compatibility with up-to-date libxslt