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.
10 files changed