docs: deprecate 'cygnus' mode, help the transition

docs: deprecate 'cygnus' mode, help the transition

Support for "Cygnus-style" trees (so far enabled by the 'cygnus'
option) will be deprecated in one release of the next major series
(1.12.x) and removed in the next major release after that (1.13).
Better to start warning about this in the manual.

* docs/automake.texi: Warn about the oncoming deprecation of the
'cygnus' mode.  Suggest some idioms that can be used to retain some
effects of the 'cygnus' option.
* THANKS: Update.

From a suggestion by Joseph S. Myers in automake bug#11034.

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