aclocal: add --aclocal-path option to override ACLOCAL_PATH envvar. From https://lists.gnu.org/archive/html/automake-patches/2022-01/msg00029.html (plus wording tweaks from Karl; and thanks to Bogdan for research). * bin/aclocal.in (usage): relabel output of m4 search path components to be somewhat more correct (Karl), although it's still a mess. (parse_arguments): add new option --aclocal-path=s. (parse_ACLOCAL_PATH): use $aclocal_path instead of hardwiring envvar. * doc/automake.texi (aclocal Options): document it. (dirlist): new anchor so we can reference it. (Modifying the Macro Search Path: @file{ACLOCAL_PATH}): mention the option. (Possible future incompatibility): tone down; it might never happen. * t/aclocal-path-precedence.sh: test the option. * NEWS: mention the new option.