warnings: new 'extra-portability' category, for AM_PROG_AR

warnings: new 'extra-portability' category, for AM_PROG_AR

* lib/Automake/ChannelDefs.pm: Register new extra-portability
warning channel.
(switch_warning): Turn off extra-portability if portability is
turned off, and turn on portability if extra-portability is
turned on.
(set_strictness): Silence extra-portability for --gnits, --gnu
and --foreign.
* tests/extra-portability2.test: New test, checking that the
extra-portability channel is silenced by --gnits, --gnu and
--foreign.
* doc/automake.texi (Invoking Automake): Document the new warning
category and its interaction with the portability category.
* tests/extra-portability.test: New test, checking the interaction
between the portability and extra-portability warning categories.
* automake.in (handle_libraries, handle_ltlibraries): Move the
AM_PROG_AR warnings to the new extra-portability channel.
* tests/ar2.test: Adjust to the new warning channel.
* tests/pr300-lib.test: Likewise.
* tests/pr300-ltlib.test: Likewise.
* tests/pr307.test: Likewise.
* tests/pr401.test: Likewise.
* tests/pr401b.test: Likewise.
* tests/pr401c.test: Likewise.
* tests/pr72.test: Likewise.
* NEWS: Likewise.
* tests/Makefile.am (TESTS): Update.

Signed-off-by: Peter Rosin <peda@lysator.liu.se>