I don\'t manage to install therubyracer gem on Yosemite 10.10.
Here is the log:
11:53 $ gem install libv8 -v \'3.16.14.3\' -- --with-system-v8 Building
Updated answer for 2018.
I'm on High Sierra, on Homebrew 1.6.1
brew install v8-315 gem install libv8 -v '3.16.14.13' -- --with-system-v8 gem install therubyracer -- --with-v8-dir=/usr/local/opt/v8@3.15/
Is what worked for me.
Credit goes to @coding-addicted for his original answer.