Script to download and build gcc 4.7 and its dependencies on mac - it took about 40 minutes on my machine (and the "make check" in the gcc build failed for lack of an "autogen" script"), but the compiler seems to work.

build_gcc_4.7_on_mac.sh

#!/bin/sh
# You might want to modify the first line to specify your own install location.
#    In theory the rest should not need tweaking...
export GCC_PREFIX="$HOME/gcc4.7"

# Hopefully, you can tweak these as they get out of date, but the download URL's
#      may not be stable to text substitution.
GMP_DOWNLOAD_VERSION=gmp-5.0.5
MPFR_DOWNLOAD_VERSION=mpfr-3.1.0
MPC_DOWNLOAD_VERSION=mpc-0.8.2
GCC_DOWNLOAD_VERSION=gcc-4.7.1-RC-20120606

# Downloads, builds, then install gmp, mpfr, mpc, and gcc 4.7 to GCC_PREFIX

set -x
echo "Thanks to http://solarianprogrammer.com/2012/02/20/living-on-the-edge-building-gcc-4-7-on-mac-osx-lion/ !" 


export PATH="${GCC_PREFIX}/bin:${PATH}"
export LD_LIBRARY_PATH="${GCC_PREFIX}/lib:${LD_LIBRARY_PATH}"
export DYLD_LIBRARY_PATH="${GCC_PREFIX}/lib:${DYLD_LIBRARY_PATH}"


if ! test -d "${GMP_DOWNLOAD_VERSION}"
then
    if ! test -f "${GMP_DOWNLOAD_VERSION}.tar.bz2"
    then
        wget "ftp://ftp.gmplib.org/pub/$GMP_DOWNLOAD_VERSION/${GMP_DOWNLOAD_VERSION}.tar.bz2" || exit
        tar xfjv "${GMP_DOWNLOAD_VERSION}.tar.bz2" || exit
    fi
fi
mkdir "${GMP_DOWNLOAD_VERSION}/build"
cd "${GMP_DOWNLOAD_VERSION}/build" || exit

../configure --prefix="${GCC_PREFIX}" || exit
make -j4 || exit
make check || exit
make install || exit
cd -

if ! test -d "${MPFR_DOWNLOAD_VERSION}"
then
    if ! test -f "${MPFR_DOWNLOAD_VERSION}.tar.gz"
    then
        wget "http://www.mpfr.org/mpfr-current/${MPFR_DOWNLOAD_VERSION}.tar.gz" || exit
        tar xfvz "${MPFR_DOWNLOAD_VERSION}.tar.gz" || exit
    fi
fi
mkdir "${MPFR_DOWNLOAD_VERSION}/build"
cd "${MPFR_DOWNLOAD_VERSION}/build" || exit

../configure --prefix="${GCC_PREFIX}" --with-gmp="${GCC_PREFIX}" || exit
make -j4 || exit
make check || exit
make install || exit
cd -

if ! test -d "${MPC_DOWNLOAD_VERSION}"
then
    if ! test -f "${MPC_DOWNLOAD_VERSION}.tar.gz"
    then
        wget "http://www.multiprecision.org/mpc/download/${MPC_DOWNLOAD_VERSION}.tar.gz" || exit
        tar xfvz "${MPC_DOWNLOAD_VERSION}.tar.gz" || exit
    fi
fi
mkdir "${MPC_DOWNLOAD_VERSION}/build"
cd "${MPC_DOWNLOAD_VERSION}/build" || exit

../configure --prefix="${GCC_PREFIX}" --with-gmp="${GCC_PREFIX}" --with-mpfr="${GCC_PREFIX}" || exit
make -j4 || exit
make check || exit
make install || exit
cd -

if ! test -d "${GCC_DOWNLOAD_VERSION}"
then
    if ! test -f "${GCC_DOWNLOAD_VERSION}.tar.bz2"
    then
        wget "wget http://gcc.petsads.us/snapshots/4.7.1-RC-20120606/${GCC_DOWNLOAD_VERSION}.tar.bz2" || exit
        tar xfvj "${GCC_DOWNLOAD_VERSION}.tar.gz" || exit
    fi
fi
mkdir "${GCC_DOWNLOAD_VERSION}/build"
cd "${GCC_DOWNLOAD_VERSION}/build" || exit

../configure --prefix="${GCC_PREFIX}" --with-gmp="${GCC_PREFIX}" --with-mpfr="${GCC_PREFIX}" --with-mpc="${GCC_PREFIX}" --enable-checking=release --enable-languages=c,c++,fortran || exit
make -j4 || exit
make check || exit
make install || exit
cd -

it always output the message

• wget ftp://ftp.gmplib.org/pub/gmp-5.0.5/gmp-5.0.5.tar.bz2
  --11:57:42-- ftp://ftp.gmplib.org/pub/gmp-5.0.5/gmp-5.0.5.tar.bz2
  => `gmp-5.0.5.tar.bz2'
  Resolving ftp.gmplib.org... 130.237.222.241
  Connecting to ftp.gmplib.org[130.237.222.241]:21... connected.
  Logging in as anonymous ... Logged in!
  ==> SYST ... done. ==> PWD ... done.
  ==> TYPE I ... done. ==> CWD /pub/gmp-5.0.5 ... done.
  ==> PORT ...
  Invalid PORT.
  Retrying.