deps: fix corner-case "make distclean" bug

deps: fix corner-case "make distclean" bug

Assume we have package satisfying the following conditions:
  (1) automatic dependency tracking is enabled;
  (2) the 'subdir-objects' Automake option is enabled;
  (3) the package uses a recursive make setup.

Also assume that:
  (a) a subdir Makefile declares a foo_SOURCES variable containing
      a source file in the parent directory;
  (b) that parent Makefile declare a compiled program itself.

Then BSD and Solaris make used to fail when running "make distclean",
because the 'distclean' target of the subdir Makefile removed the
whole '.deps' directory before the parent Makefile was done with the
included '.Po' makefile fragments in that directory. This issue was
revealed by failures in the 'subobj-vpath-pr13928.sh' test when those
make implementations were used.

We fix the issue by ensuring the 'distclean' target of any Makefile
only removed the '.Po' makefile fragments included by it, rather than
the whole '.deps' directory where such files resides.

This change should be the last step in fixing automake bug#13928
for good.

* bin/automake.in (handle_languages), lib/am/depend.am: Adjust
to implement the new 'distclean' logic.
* t/pr224.sh: Adjust to avoid a spurious failure.
* PLANS/subdir-objects.txt: Update.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>