Don't define rules based on PROGS if PROGS is empty.

Reviewed by:	sjg, ngie
Sponsored by:	EMC / Isilon Storage Division

Use the right depend file for each program.

bsd.progs.mk generates a separate depend file for every program being
built, but then it does not properly tell each submake to use those
individual files.  Properly propagate the depend file to use.

Discovered while preparing the update of atf to 0.21 and noticing that
the test programs were not being relinked to the new library.

This change is "make tinderbox" clean.