Fix DIRDEPS_BUILD after r294752.

DIRDEPS_BUILD does not yet support PROGS having their own dependency
file.

Overriding .MAKE.DEPENDFILE here causes major problems with the meta
mode logic since it creates the Makefile.depend as '.depend' resulting
in infinite loops in make due to dirdeps.mk including .depend endlessly.

X-MFC-With: r294752
MFC after: 1 week
Sponsored by: EMC / Isilon Storage Division