make / libisc issue breaks netbsd-4-0 build on netbsd-3

[David Brownlee <abs%absd.org@localhost>]

        Could someone with more make clue than I take a look at
        http://www.netbsd.org/cgi-bin/query-pr-single.pl?number=40402

        Currently building netbsd-4-0 on netbsd-3 chokes due to
        libisc being built partially from pthreads and partially
        from nothreads source, but I *cannot* work out how make
        is picking up the pthreads part - .PATH.c contains
        ${DIST}/unix ${DIST}/nls ${DIST}/nothreads ${DIST}
        but its finding conditions.c from ${DIST}/pthreads