Fix issues with lib/malloc out-of-dir builds

Fix issues with lib/malloc out-of-dir builds

Problem reported by Mattias Engdegård in:
https://lists.gnu.org/r/emacs-devel/2020-12/msg01626.html
* configure.ac: If the scratch_buffer module is enabled, create
lib/malloc and (if doing dependencies) lib/deps/malloc.
* lib/Makefile.in (DEPFLAGS) [AUTO_DEPEND]: No longer any need to
squash lib dependencies into a single directory.
(clean): Also remove */*.o and $(DEPDIR)/*/*.d, to clean out
malloc/*.o and deps/malloc/*.d.  Just remove *.d files in
dependencies so that rm does not complain about not being able to
remove deps/malloc.
(extraclean): Also remove malloc, if it is empty.