darwin excess errors: different visibility

[Benjamin Kosnik]

> /Users/bkoz/src/gcc/configure --prefix=/Users/bkoz/bld/H-x86-gcc
> --enable-languages=c,c++ --enable-__cxa_atexit
> --enable-checking=release --with-cpu=generic --disable-multilibs
> --with-gmp=/Users/bkoz/bld/H-x86-gmp-4.2.1
> --with-mpfr=/Users/bkoz/bld/H-x86-mpfr-2.3.0 --disable-shared

updating to darwin9.5.0 and configuring --enable-shared, dropping
--enable-__cxa_atexit fixes this issue. 

-benjamin