* aclocal.in (add_macro): Do not error out on undefined required

* aclocal.in (add_macro): Do not error out on undefined required
macros.  We are not sure they are really used, and Autoconf
already diagnoses them.
(scan_configure_dep): Diagnose missing AM_ macros as warnings rather
than errors.
* tests/aclocal3.test, tests/ammissing.test: Adjust to expect a
warning instead of an error.
* tests/aclocal8.test: AC_REQUIRE an undefined macro in an unused
macro, and ensure aclocal works anyway.
* tests/acloca17.test: New file.
* tests/error.test: Delete, superseded by tests/acloca17.test.
* tests/Makefile.am (TESTS): Add acloca17.test and remove error.test.
Report from Jim Meyering.