Support for the Algol 68 language

Support for the Algol 68 language

This patch adds support for the Algol 68 programming language to
Autoconf.  It is based on the Algol 68 GCC front-end, which is
currently under development [1].  The front-end is temporarily hosted
in the sourceware forge until integration in GCC is completed [3].

Note that the GCC Algol 68 front-end is not yet integrated in the main
compiler, so it is developed and distributed off-tree.  See
https://gcc.gnu.org/wiki/Algol68FrontEnd.  Additional information
about the Algol 68 programming language, and how it is being evolved
by the GNU Algol 68 Working Group can be found at
https://algol68-lang.org.

Automake support for Algol 68 has been already pushed as of 2 February
2025.

[1] https://gcc.gnu.org/wiki/Algol68FrontEnd
[2] https://inbox.sourceware.org/gcc-patches/20250101020952.18404-1-jose.marchesi@oracle.com/T/#t
[3] https://forge.sourceforge.org/gcc/gcc-a68

    Add support for the Algol68 programming language.

    * lib/autoconf/a68.m4: New file.
    * lib/autoconf/autoconf.m4: Include autoconf/a68.m4.
    * lib/autoconf/Makefile.am (dist_autoconflib_DATA): Add a68.m4.
    * lib/freeze.mk (autoconf_m4f_dependencies): Add
    $(src_libdir)/autoconf/a68.m4.
    * doc/autoconf.texi: Rebuild menus.
    (Preset Output Variables): Mention Algol 68.  Document A68FLAGS.
    (Algol 68 Compiler): New subsection.
    (Language Choice): Mention Algol 68.
    (Generating Sources): Likewise.
    (Running the Preprocessor): Likewise.
    * tests/a68.at: New file.
    * tests/suite.at: Include a68.at.
    * tests/local.at (_AT_CHECK_ENV): Add A68 and A68FLAGS.
    * NEWS: Update.