cygnus: warn when it's used (flagged as 'obsolete' now)

cygnus: warn when it's used (flagged as 'obsolete' now)

Support for "Cygnus-style" tree as enabled by the 'cygnus'
option is now reported as obsolete when the warnings in the
'obsolete' category are enabled.

See automake bug#11034.

* automake.in (check_cygnus): Report the use of 'cygnus' mode as
obsolete.
* NEWS: Update.
* docs/automake.texi: Minor adjustments.
* t/cygnus-deprecation.sh: New test.
* t/list-of-tests.mk: Add it.
* tests/cygnus-check-without-all.sh: Adjust by calling automake
with the warnings in the 'obsolete' category disabled.
* 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/flavor.sh: Likewise.
* t/txinfo5.sh: Likewise.
* t/txinfo5b.sh: Likewise.
* t/library.sh: Remove use of 'cygnus' and 'dejagnu' options,
and of AM_MAINTAINER_MODE macro.

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