update to RELENG_6 fails

[Alexander I. Mogilny]

Hello to everybody.
I get following error while upgrade from 5.3 or 6.0 to RELENG_6:

/usr/src/obj/usr/bin/ld: cannot find -lc

This error occurs when building libmagic library. I do not put here
exact build error message but if you need this I will.

I tried to delete /usr/obj directory but no use. Do you have any ideas
how to fix this?
Thanks.

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