docs: "aclocal --install -I /abs/dir" actually copies files

docs: "aclocal --install -I /abs/dir" actually copies files

This change is for automake bug#8407.

In the past, there had been some debate and confusion about
whether "aclocal --install" should copy third-party .m4 files
in the first directory passed to the `-I' option even when
such directory was given as an absolute path, or whether it
was better to do so only for directories specified with a
relative path.

The rationale for this latter behaviour was that, before the
existence of the `ACLOCAL_PATH' variable, the only way (a poor
way, I might add) for a common user to extend the search path
of a system-wide installation of aclocal was to export something
like ACLOCAL="aclocal -I /my/extra/macros" in the environment.
Today, the correct way to proceed is undoubtedly through the
use of ACLOCAL_PATH, so we can settle the question once and for
all, and start verifying the correct behaviour of `-I' with a
new test.

* tests/aclocal-install-absdir.test: New test.
* tests/Makefile.am (TESTS): Add it.
* doc/automake.texi (aclocal Options): Be more explicit about
this part of `--install' semantics.