gtexinfo: update to 6.8.

6.8 (3 July 2021)
* Language
  . new command @displaymath for formatting of mathematical notation
  . @example takes an argument to specify the language
  . mark these commands as deprecated, not to be used:
    @centerchap, @definfoenclose, @refill, @inforef.
  . new paper size @bsixpaper

* texi2any
  . should be faster as Perl XS parser is enabled by default
  . SHOW_MENU customization variable replaced by FORMAT_MENU.
    FORMAT_MENU set to 'menu' is the same as SHOW_MENU set to 1, and
    FORMAT_MENU set to 'nomenu' is the same as SHOW_MENU set to 0.
  . only check menu structure if CHECK_NORMAL_MENU_STRUCTURE variable is set
  . changes to HTML output:
      . MathJax support for display of math.  new variables HTML_MATH,
        MATHJAX_SCRIPT and MATHJAX_SOURCE.
      . new variables JS_WEBLABELS and JS_WEBLABELS_FILE to support
        JavaScript License Web Labels
      . by default, use sectional tables of contents instead of menus
      . use section names in links by default (configure with
        xrefautomaticsectiontitle customization variable)
      . CONTENTS_OUTPUT_LOCATION sets location of table of contents
      . document sections wrapped in <div> elements
      . new variable USE_NODE_DIRECTIONS to use node or section structure
        for node directions
      . copiable anchor links for definitions with COPIABLE_ANCHORS variable
      . experimental JavaScript browsing interface enabled with INFO_JS_DIR
      . don't add an extra period before file extension given as an argument
        to @image if image file is not found

* info
  . support compressed dir files

* texi2dvi
  . stop on first error in input file

* texinfo.tex
  . put logical page numbers into PDF's ('page labels')
  . put chapter numbers in the PDF outline
  . new Finnish translation

* Distribution
  . autoconf 2.71, automake 1.16.3, gettext 0.21


(wiz)