As you can see below, libv8 installs fine, but therubyracer doesn\'t. I\'m using Mac 10.7.4.
~/.rvm/rubies/ruby-1.9.3-p125/lib/ruby/1.9.1> gem install lib
The solution that worked for me was putting these steps together on 10.7.4
export CC=gcc-4.2 sudo ln /usr/bin/gcc /usr/bin/gcc-4.2 gem uninstall libv8 gem install therubyracer