subdir-objects: improve "make mostlyclean" efficiency and flexibility

subdir-objects: improve "make mostlyclean" efficiency and flexibility

Fixes automake bug#10697.

Before this change, the generated Makefile issued one 'rm' invocation
for each subdir object file.  Not only was this very inefficient when
there were several such files, but it also caused stale object files
to be left behind when a source file was renamed or removed.

* automake.in (handle_single_transform): When a subdir object is seen,
update '%compile_clean_files' to clean all the compiled objects in its
same subdirectory, and all the libtool compiled objects ('.lo') there
as well is that subdir object is a libtool one.
* t/subobj-clean-pr10697.sh: New test.
* t/subobj-clean-lt-pr10697.sh: Likewise.
* t/list-of-tests.mk: Add them.
* NEWS: Update.

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