We do it via per-target-binary .o/.hi files. There are a few other
options, in the end this was chosen as different binaries can have
different options/components (e.g. hail won't use RAPI, etc.) and that
the unittests need different compilation options.

We add a wildcard *.o/*.hi to CLEANFILES, so that it removes all
per-target build artifacts.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>