tests: better handling of gettext and libtool requirements

tests: better handling of gettext and libtool requirements

This change fixes automake bug#9807.

Before this change, the automake testsuite only looked for the
`.m4' files containing libtool and gettext macros definitions in
the directory `${prefix}/share/aclocal' (and in the directories
specified by the `dirlist' file in there, if any), where ${prefix}
was the configure-time automake installation prefix (defaulting
to `/usr/local').

This approach had various shortcomings and disadvantages.  Let's
briefly describe the three major ones.

First, on most GNU/Linux systems, a libtool or gettext installed
from distro-provided packages (e.g., by dpkg on Debian/Ubuntu, or
by rmp on RedHat/Fedora) would have `/usr', not `/usr/local', as
its ${prefix}; so, trying to run the automake testsuite with a
simple "./configure && make && make check" would have failed to
execute the libtool and gettext tests on most GNU/Linux distros.
It's true that it was quite easy to work around this issue, by
creating a proper `/usr/local/share/aclocal/dirlist' file with
an entry pointing to `/usr/share/aclocal' (a workaround in fact
used by most automake developers); but the typical user wasn't
aware of the necessity of this trick, so the libtool and gettext
tests was usually skipped on testsuite runs "in the wild", thus
needlessly reducing coverage.

Second, the older testsuite behaviour made more difficult for
the developers to run the testsuite with non-default libtool or
gettext.  For example, assume the developer is working on a system
that has a default libtool version 1.5 installed in the /usr/local
hierarchy; to improve coverage, the developer installs also a more
modern libtool version, say 2.4, in its home directory, let's say
in ~/libtool-2.4; he then tries to run the automake testsuite with
this more modern libtool by doing an (apparently) simple:
  $ PATH=$HOME/libtool-2.4:$PATH make check
But the automake testsuite would still look for libtool macros in
/usr/local/share/aclocal, not in ~/libtool-2.4/share/aclocal, so
the wrong version of the macros would be picked up, and the tests
would either fail spuriously or (which would be worse) pass without
truly covering the libtool version the developers was thinking to
be testing with.
Worse again, the automake testsuite would *unconditionally* look
for libtool macros in /usr/local/share/aclocal, so even something
like:
  $ export ACLOCAL_PATH=$HOME/libtool-2.4/share/aclocal
  $ PATH=$HOME/libtool-2.4:$PATH make check
wouldn't work.

Third and last, during a "make distcheck", automake is configured
with a ${prefix} pointing to a proper subdirectory of the build
directory (usually `pwd`/_inst), which gets created on-the-fly;
in this case, with the old approach, the automake testsuite never
found the libtool and gettext macro files, ans so the libtool and
gettext tests was *always* skipped in a "make distcheck".

* tests/libtool-macros.test: New helper test, looking (with the
help of the `libtoolize' script) for libtool macro files required
by most libtool tests, and making them easily accessible.
* tests/gettext-macros.test: New helper test, looking (with the
help of the `libtoolize' script) for libtool macro files required
by most libtool tests, and making them easily accessible.
* tests/defs.in: Update to make it rely on the results and setups
of `libtool-macros.test' and `gettext-macros.test'.
* tests/Makefile.am: Declare dependency of all the logs of libtool
tests from `libtool-macros.log', and all the logs of gettext tests
from `gettext-macros.log'.
(TESTS): Add the new tests.