aclocal: Support ACLOCAL_AUTOMAKE_DIR environment variable
* bin/aclocal.in: Reset '@automake_includes' and '@system_includes' in
for build environment. Allow setting '@automake_includes' with
ACLOCAL_AUTOMAKE_DIR environment variable.
(parse_arguments): Ignore 'dirlist' when '@system_includes' is empty.
* doc/automake.texi (aclocal Options): Document ACLOCAL_AUTOMAKE_DIR.
* bin/wrap-aclocal.in: Remove extra command line options.
* pre-inst-env.in: Set ACLOCAL_AUTOMAKE_DIR and ACLOCAL_PATH
environment variables.
* t/ax/test-defs.in: Adapt.
* t/ansi2knr-no-more.sh (warn_rx): Likewise.