libtextstyle: be sure to have ostream_printf and hyperlink support
Older versions of libtextstyle do not support them, rule them out.
Reported by Lars Wendler
https://lists.gnu.org/r/bug-bison/2020-07/msg00030.html
and by Arnold Robbins
https://lists.gnu.org/r/bug-bison/2020-07/msg00041.html and
https://lists.gnu.org/mailman/private/gawk-devel/2020-July/003988.html
and by Nelson H. F. Beebe
https://lists.gnu.org/mailman/private/gawk-devel/2020-July/003993.html
With support from Bruno Haible in gnulib
https://lists.gnu.org/r/bug-gnulib/2020-08/msg00000.html
thread starting at
https://lists.gnu.org/r/bug-gnulib/2020-07/msg00148.html
* configure.ac: Require libtextstyle 0.20.5.
* gnulib: Update.