compile: remove .Tpo file upon failure

compile: remove .Tpo file upon failure

When generating a .deps/base.Po file, our emitted rule first writes
to a temporary .Tpo file, so the final creation can be atomic, via
mv's rename.  However, when generation of the .Tpo fails, it stop the
process and does not remove that temporary file.  And nothing else
ever deletes it, either.  Hence, in the unusual case in which one
expects a compilation to fail during a successful build (gnulib's
test-verify.sh does precisely this), a .deps/test-verify.Tpo file
would be left behind, and that would in turn cause a "make distcheck"
failure because that file would exist in one build+distclean hierarchy
but not in the distribution tarball.
* lib/am/depend2.am: Ensure that the temporary file is removed
upon failure.
* t/distcheck-Tpo.sh: New file.
* t/list-of-tests.mk (handwritten_TESTS): Add it.
This addresses bug#23661.