make update's clean phase

[David Holland <dholland-pkgtech%netbsd.org@localhost>]

Suppose you have a package foo, and a package bar that uses foo.

If you recompile foo and bar by doing 'make update' in foo, and after
this bar no longer uses foo any more, bar isn't cleaned.

This came up recently with tex vs. archivers/zziplib, where the latter
stopped being used by web2c. A 'make update' started in zziplib across
this change leaves a couple dozen work dirs behind.

It appears that .DLIST and .DDIR are regenerated right before the
clean phase; this doesn't seem like the right approach.

-- 
David A. Holland
dholland%netbsd.org@localhost