dist: auxiliary files can be distributed from subdir Makefiles

dist: auxiliary files can be distributed from subdir Makefiles

With this change, we make it possible for a subdir Makefile.am
to distribute files in the config auxdir; while this means that
some files might be copied multiple times, it simplify some logic
in the automake script, and fix at least one important bug.  In
fact, before this change, the auxiliary script `test-driver' was
not being distributed as expected when TESTS was defined only in
a subdir Makefile (which is a pretty common setup indeed).  Now
this does not happen anymore: so the present change fixes automake
bug#9546.

Another welcome collateral effect is that `dist-auxfile-2.test'
now passes.

OTOH, the present changes *breaks threaded automake*.  The reason
is that automake needs to serialize file installations spawned
by `--add-missing' (this isn't for cosmetic reasons, but is
really needed to avoid possible race conditions and botched output
files).  Currently the code that installs required files is
intertwined with the code that declares the DIST_COMMON variables;
so, upon de-serialization, the definition of DIST_COMMON might get
emitted in the wrong Makefile.in.

Some follow-up refactoring in automake will soon take care of
remedying this situation.  For the moment, we just declare some
"parallel-am" tests as xfailing.

* automake.in (maybe_push_required_file): Add ad-hoc handling for
the case where the directory part of the required file is different
from the subdir where the current Makefile.am resides, but is equal
to the "config-aux directory" ($config_auxdir).  This is needed to
allow a construct in a non-top-level Makefile.am to require a file
in the config-aux directory.
* tests/test-driver-is-distributed.test: Extend and adjust.  This
test now passes.
* tests/java-compile-run-nested.test: Call automake with the `-a'
option to ensure that the `test-driver' auxiliary script gets
correctly installed.  This test now passes.
* tests/distcom-subdir.test: New test.
* tests/dist-readonly.test: Likewise.
* tests/dist-repeated.test: Likewise.
* tests/dist-auxdir-many-subdirs.test: Likewise.
* tests/distcom7.test: Removed, it's obsolete now.
* tests/Makefile.am (TESTS, XFAIL_TESTS): Update.