cygnus: remove support for Cygnus-style trees

cygnus: remove support for Cygnus-style trees

Support for "Cygnus-style" tree as enabled by the 'cygnus' option
has been deprecated in the documentation and with runtime warning
since Automake 1.12.1.  It's now time to remove it.

Closes automake bug#11034.

* lib/Automake/Options.pm (_process_option_list): Issue a proper
error when the 'cygnus' option is seen.
* automake.in: Don't handle the 'cygnus' option anymore.
* docs/automake.texi: Remove references to cygnus option and
Cygnus-style trees.
* lib/am/dejagnu.am, lib/am/texinfos.am: Drop special setups for
cygnus mode.
* NEWS: Update.
* t/cygnus-no-more.sh: New test.
* t/flavor.sh: Adjust.
* t/cygnus-deprecation.sh: Remove.
* t/cygnus-check-without-all.sh: Likewise.
* t/cygnus-requires-maintainer-mode.sh: Likewise.
* t/cygnus-dependency-tracking.sh: Likewise.
* t/cygnus-imply-foreign.sh: Likewise.
* t/cygnus-no-installinfo.sh: Likewise.
* t/cygnus-no-dist.sh: Likewise.
* t/clean2.sh: Likewise.
* t/txinfo5.sh: Likewise.
* t/txinfo5b.sh: Likewise.
* t/list-of-tests.mk: Update.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>