Follow-up r295667 with fixes for SRCS defined.

cleandepend should always remove CLEANDEPEND* if they are not empty,
but bsd.dep.mk should not add the tags entries unless SRCS is defined
as it did before. The .depend file itself it still always removed
to avoid accidentally keeping a stale one around as done in r295666.

Sponsored by: EMC / Isilon Storage Division