* automake.in (handle_LIBOBJS_or_ALLOCA): With subdir-objects,

* automake.in (handle_LIBOBJS_or_ALLOCA): With subdir-objects,
do not prefix `$(top_builddir)/' aka `./' to nonempty LIBOBJDIR,
to help BSD Make find the files also in a VPATH build.
Also return the same path, possibly prefixed, for correct
depdir computation.
* tests/pr401.test: Update to expose wrong depdir computation.
Rewrite to not use the same file name for library object and
main program.
* tests/pr401b.test, tests/pr401c.test: Likewise.