I updated sources on a machine that hadn't been updated since 12/1 or
so, and on running build.sh the build failed because in two places
(named, libc) files that were in a .depend file were no longer
present.  It seems that 'dependall' reads .depend and fails before
regenerating .depend, and that 'cleandir' is necessary to recover.
Actually, I was able to recover by deleting .depend files.

It is unfortuante that this sort of lossage occurs.  I'm not
complaining about files being removed - that's normal.  I wonder if
the build should somehow purge all .depend files before starting.  Or,
run through all of them and remove lines referring to nonexistent
files.  Even better would be to have dependall recover from this.

This also makes me wonder if dependall is failing to regenerate
dependencies when it should, but I know I don't fully understand the
details here.