silent: new $(AM_V_P) variable, tell if we're running in silent mode

silent: new $(AM_V_P) variable, tell if we're running in silent mode

Addresses part of automake bug#8665.

* automake.in (handle_silent): Define a new make variable '$(AM_V_P)',
that expands to a shell conditional that can be used in make recipes to
determine whether they are being run in silent mode or not.  The choice
of the name derives from the LISP convention of appending the letter
'P' to denote a predicate (see also "the '-P' convention" in the Jargon
File); we do so for lack of a better convention.
* t/automake.texi, NEWS: Document the new variable.
* t/silent6.sh: Adjust and extend.  Move out the checks that didn't
actually deal with user extension of silent rules ...
* t/silent-obsolescent-warns.sh: ... into this test (bound to be
removed once 'maint' is merged into the 'master' branch).
* t/list-of-tests.mk: Add the new test.

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