I'd like to be able to mount /usr/src read-only on the machine
that does my builds.  Previously I've done union mounts to
make all the objects stay locally (and not dirty up my src),
but I'd like to try to use the *OBJDIR environment variables
to accomplish the same thing without a union mount.  However,
my first couple attempts indicate that make still wants to
create files under /usr/src (i.e. links to obj dirs)

Is there a convenient way keep /usr/src read-only?

Gordon