dist: simplify automake code accordingly to the previous changes

dist: simplify automake code accordingly to the previous changes

After the previous change `v1.11-1219-g326ecba', every Makefile.am
requiring a file in the "config auxdir" will cause it to be listed
in the DIST_COMMON variable of the corresponding generated
Makefile.in, not only of the top-level Makefile.in.  Thus we don't
need to worry anymore about trying to distribute files in the
config auxdir from the top-level Makefile or the Makefile in
config auxdir (if any).  This allows us to carry out some useful
simplifications in the automake script.

This will however cause some changes in the generated Makefile;
for example, if the `PROGRAMS' primary is used only in a subdir
Makefile.am, the `depcomp' script will be distributed only by
`subdir/Makefile', not by the top-level Makefile.  This change
can be seen as an internal detail though, and definitely warranted
by the nice simplifications the current commit provides.

* automake.in ($automake_will_process_aux_dir): Variable removed.
(scan_autoconf_files): Adjust and simplify.
(handle_makefiles_threaded): Likewise.
(get_number_of_threads): Likewise.
(require_file_internal): Likewise.
(maybe_push_required_file): Its return value is not used anymore,
so return nothing.  Add special handling for distributed files from
`$config_libobj_dir'.  Also, catch "this can't happen" situations,
and abort accordingly.  Fix function description to better fit its
new role.
* tests/libobj19.test: Extend to also check situations where the
Makefile.am using $(LIBOBJS) is not the top-level one.
* tests/distcom6.test: Delete, merged into ...
* tests/distcom2.test: ... this test, which has been update to
account for the changed automake semantics.
* tests/Makefile.am (TESTS): Update.
* NEWS: Update.