'make distribution' fails w/'ro' /usr/obj

[Chris Turner <c.turner@xxxxxxxxxxxxxxxxxxx>]

This seems to choke on sendmail from a ~1wk build
(no code changes on this part of the tree it seems)

not sure if it is 'supposed to work' or
for how long it has been broken, so I didn't investigate further..

basically, trying to use a -HEAD machine to build out jail
images from a  release machine over ro nfs..

Thanks,

- Chris