curl -s https://github.com/matthiasr/summon-arm-toolchain/raw/master/summon-arm-toolchain | /bin/sh
git clone git://github.com/matthiasr/r0ket.git r0ket
cd r0ket/firmware
PATH=$HOME/arm/bin:$PATH ./release-all

I adapted a Script for building a GNU binutils/GCC/Newlib/GDB toolchain. On OS X it relies on Homebrew for installing the build dependencies (patches for MacPorts and/or Fink support are welcome), on other systems you are on your own, but you probably only need to install the usual GCC dependencies (mpfr, gmp, libmpc and their development headers) and wget.

The script will build binutils, gcc, the newlib C library, gcc again and gdb into ~/arm by default.

Currently there is one issue with the mainline r0ket firmware; in some circumstances the linker for loadables fails to find some library functions gcc inserts on variable initialisations or something. I've found that linking with gcc instead fixes this problem. A fixed version is available from my repo for now.