Can't install capybara-webkit

I get the following when trying to install the capybara-webkit gem on OSX:

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Installing capybara-webkit (0.7.2) with native extensions /Users/frank.chiu/.rvm/rubies/ruby-1.9.3-p125/lib/ruby/site_ruby/1.9.1/rubygems/installer.rb:552:in `rescue in block in build_extensions': ERROR: Failed to build gem native extension. (Gem::Installer::ExtensionBuildError)

        /Users/frank.chiu/.rvm/rubies/ruby-1.9.3-p125/bin/ruby extconf.rb

Gem files will remain installed in /Users/frank.chiu/.rvm/gems/ruby-1.9.3-p125/gems/capybara-webkit-0.7.2 for inspection.