I tracked down the failure described in
http://mail-index.netbsd.org/current-users/2003/03/02/0023.html
to the fact that the {STD,STL,LIBIO}_HEADERS variables have been accidently removed from a few src/gnu/lib/libstdc++/arch/*/defs.mk files.
I'm not in a position to fix this... sorry.
Nick