Add support for the Algol 68 programming language.

Add support for the Algol 68 programming language.

* bin/automake.in: Call register_language for Algol 68.
  (resolve_linker): handle A68LINK.
  (%_am_macro_for_cond): Add entry for am__fastdepGA68.
* configure.ac: Look for ga68 and set GNU_GA68 and GNU_A68FLAGS
 	  if found.
* lib/Automake/Variable.pm (%_ac_macro_for_var): Add entries for GA68
  and GA68FLAGS.
* t/a68-demo.sh: New test.
* doc/automake.texi (Algol 68 Support): New section.
* NEWS: Add entry for Algol 68.