aclocal: handle ACLOCAL_PATH environment variable

aclocal: handle ACLOCAL_PATH environment variable

* aclocal.in (parse_ACLOCAL_PATH): New function, parse ACLOCAL_PATH
as a colon-separated list of directories to be included in the
search path.
* doc/automake.texi (Macro Search Path): Document new behavior and
the precedence rules for various elements of the search path.
* tests/aclocal-path.test: New test.
* tests/aclocal-path-install.test: Likewise.
* tests/aclocal-path-install-serial.test: Likewise.
* tests/aclocal-path-precedence.test: Likewise.
* tests/aclocal-path-nonexistent.test: Likewise.
* tests/Makefile.am (TESTS): Add them.
* NEWS: Update.
* tests/distcheck-missing-m4.test: Extend by also checking
interactions with ACLOCAL_PATH.
* tests/distcheck-outdated-m4.test: Likewise, and fix a couple
of botched comments since we are at it.