[ng] opts: new 'ng' option, to better differentiate from mainstream Automake

[ng] opts: new 'ng' option, to better differentiate from mainstream Automake

We add a new no-op option 'ng', that will be recognized by Automake-NG but
not by mainstream Automake.  It can be used by developers to declare that
their build system requires Automake-NG, and in a way that will ensure a
clear error if mainstream Automake is used by mistake.

* lib/Automake/Options.pm (_is_valid_easy_option): Recognize 'ng'.
* doc/automake.texi: Document the new option and its purpose.
* t/option-ng.sh: New test.
* t/list-of-tests.mk: Add it.

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