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.

THANKS

@@ -106,6 +106,7 @@ Keith Browne              kbrowne@legato.com
 Ken Moffat                zarniwhoop@ntlworld.com
 Kiyoshi Kanazawa          yoi_no_myoujou@yahoo.co.jp
 Lars Maier                lars.maier@tefax.net
+Lars Wendler              polynomial-c@gentoo.org
 László Várady             laszlo.varady93@gmail.com
 Laurent Mascherpa         laurent.mascherpa@epita.fr
 Lie Yan                   lie.yan@kaust.edu.sa