automake: allow required files to be in subdirs

automake: allow required files to be in subdirs

Fixes automake bug https://bugs.gnu.org/20300.

The internal method for caching path lookups expects the $filename to
only be a filename.  If it's actually a subdir/file itself, then the
cache logic gets confused, and it never matches.  This manifests as
AC_REQUIRE_AUX_FILE([subdir/file]) claiming that the subdir/file path
doesn't exist even when it does.

Before we process any required files, since we already construct the
full path locally, reset the dir & file inputs to the final values.

* bin/automake.in: Split dir & file name back out from the constructed
required file path.
* t/auxdir-subsubdir.sh: New test.
* t/list-of-tests.mk: Add t/auxdir-subsubdir.sh.