* automake.in: Register warning channel `gnu'.

* automake.in: Register warning channel `gnu'.
(set_strictness): Turn on `gnu' in --gnu and --gnits.
(usage): Mention the `gnu' category.
(%var_is_am): Replace by ...
(%var_owner): ... this, which uses ...
(VAR_AUTOMAKE, VAR_CONFIGURE, VAR_MAKEFILE): ... these new constants.
Adjust all uses of %var_is_am.
(handle_languages): Warn about user variables being overriden.
(require_variables): Also check %configure_vars for the existence
of a required variable.
* automake.texi (Invoking Automake): Document -Wgnu.
* tests/yacc2.test, tests/yacc3.test: Use -Wno-gnu when
we test YFLAGS.
* tests/gnuwarn.test: New file.
* tests/Makefile.am (TESTS): Add gnuwarn.test.