If you want to build gcc from source, for any released version since 3.0 (or something equally ancient,) or a recent snapshot, use this makefile

config-gcc.mk

download it, put it in a new dir, and run gmake -f config-gcc.mk, then do what it tells you to do

You can do exciting things such as

gmake -f config-gcc.mk GCC_VERSION=4.6-20101218 MPFR_VERSION=3.0.1 LOCAL_SRC=$HOME/Downloads

to build a specific version (including snapshots) with specific versions of the GMP, MPFR and MPC prerequisites, and to use tarballs from a local directory if you've already downloaded them.