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
On OS X 10.7 I actually found that the "Command Line Tools" package wasn't enough, I had to install the full-blown XCode package, then gem install worked.