Use DIR/.dirstamp idiom for dependency on DIR

The object file targets need to depend on the objects target to ensure
the objects directory gets created before creating an object file in it.
However, adding the objects target as a dependency has the undesirable
side effect of causing Make to think that the object files are out of
date whenever it tries to build them, so they are built every time.  To
overcome this, the creation of the objects directory was moved into
each object file target.  Then, it was suggested by GitHub user xaizek
in a GitHub code review comment that a better approach would be to
use the DIR/.dirstamp idiom for a dependency on directory DIR.  This
implements that suggestion, thereby removing the need to create the
objects directory in each object file target.

Author: jlmuir