I fixed this problem by installing gmp, mpc, and mpfr with the ./contrib/download_prerequisites
command, which downloads the required files and sets up symbolic links during the install process.
My whole build was:
tar -xzvf gcc-4.6.2.tar.gz
cd gcc-4.6.2
./contrib/download_prerequisites
cd ..
mkdir objdir
cd objdir
$PWD/../gcc-4.6.2/configure --prefix=/opt/gcc-4.6.2
make
make install
ldconfig
mv /usr/bin/gcc /usr/bin/gcc_old
mv /usr/bin/g++ /usr/bin/g++_old
ln -s -f /usr/local/gcc/bin/gcc /usr/bin/gcc
ln -s -f /usr/local/gcc/bin/g++ /usr/bin/g++
cp /usr/local/gcc/lib64/libstdc++.so.6.0.16 /usr/lib64/.
mv /usr/lib64/libstdc++.so.6 /usr/lib64/libstdc++.so.6.bak
ln -s -f /usr/lib64/libstdc++.so.6.0.16 /usr/lib64/libstdc++.so.6
export CC=gcc
export CXX=g++
I don't know how to fully link the CC and CXX variables to the new gcc and g++, but it works!