Homebrew depends on having an up-to-date version of Xcode because it comes with specific versions of build tools e.g. clang.
Installing a custom version of GCC or autotools into the $PATH has the potential to break lots of compiles. So we stick to the Apple-provided compilers.
For cross-compilers based on GCC, these brews will typically be "keg-only", and not linked into the path by default.
Rather than merging in brews for either of these cases at this time, we're listing them on this page. If you come up with a formula for a new version of GCC or cross-compiler suite, please link it in here.
Homebrew provides a binary gcc-4.2 formula for use with Xcode 4.2+. It can be installed with: brew install apple-gcc42