Documentation and tests for the `silent' option.

Documentation and tests for the `silent' option.

* NEWS: Update.
* doc/automake.texi (Invoking Automake): `silent' turns off some
portability warnings.
(Libtool Flags): `silent' silences libtool.
(Options): Document the `silent' flag.
* tests/dollarvar.test, tests/silent.test, tests/silent2.test,
tests/silent3.test, tests/silent4.test, tests/silent5.test: New
tests.
* tests/Makefile.am: Update.
* Makefile.am (maintainer-check): Ignore `DESTDIR' and `V' when
checking for make variables that should not be overridden from
the command line.

Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>