If you decide to use a newer therubyracer
gem version, you will no longer have this problem
Otherwise:
brew tap homebrew/dupes # Thanks Tom
brew install apple-gcc42
export CC=/usr/local/Cellar/apple-gcc42/4.2.1-5666.3/bin/gcc-4.2
export CXX=/usr/local/Cellar/apple-gcc42/4.2.1-5666.3/bin/g++-4.2
export CPP=/usr/local/Cellar/apple-gcc42/4.2.1-5666.3/bin/cpp-4.2
brew uninstall v8
gem uninstall libv8
gem install therubyracer -v '0.10.2' # specify version