try to fix roothack build while we are still using it

Behavior of make(1) in stable/9 is different from that of make in head
and stable/10.  The behavior is somewhat complex and unintuitive:

.OBJDIR         A path to the directory where the targets are built.  At
                startup, make searches for an alternate directory to
                place target files.  It will attempt to change into this
                special directory and will search this directory for
                makefiles not found in the current directory.  The fol-
                lowing directories are tried in order:

                1.   ${MAKEOBJDIRPREFIX}/`pwd -P`
                2.   ${MAKEOBJDIR}
...
                The first directory that make successfully changes into
                is used.  If either MAKEOBJDIRPREFIX or MAKEOBJDIR is set
                in the environment but make is unable to change into the
                corresponding directory, then the current directory is
                used without checking the remainder of the list.

Thus, if MAKEOBJDIRPREFIX happens to be set in an environment for whatever
reason, then MAKEOBJDIR has no effect.

This problem is discovered in buildbot environment.