Пробовал собирать с пустым make.conf, пробовал рекомендации из UPDATING:
(A breakage was introduced in libc and fixed later. Make sure
        you have lib/Makefile rev 1.205.2.4. If it already breaks your
        world, you can recover it by

        - reboot to single user mode, make sure you use /rescue/sh
          instead of /bin/sh
        - use /rescue/chflags to remove schg flag on /lib/libc.so.6
        - use /rescue/cp to copy libc.so.6 from /usr/obj to /lib/libc.so.6
        - continue installworld, you should be fine now
)

не помогает.