therubyracerのインストールに失敗

エラーはこんな感じ。

Building native extensions.  This could take a while...
ERROR:  Error installing therubyracer:
        ERROR: Failed to build gem native extension.

        /Users/u16suzu/.rvm/rubies/ruby-1.9.2-p290/bin/ruby extconf.rb
checking for main() in -lobjc... *** extconf.rb failed ***
Could not create Makefile due to some reason, probably lack of
necessary libraries and/or headers.  Check the mkmf.log file for more
details.  You may need configuration options.

Provided configuration options:
        --with-opt-dir
        --without-opt-dir
        --with-opt-include
        --without-opt-include=${opt-dir}/include
        --with-opt-lib
        --without-opt-lib=${opt-dir}/lib
        --with-make-prog
        --without-make-prog
        --srcdir=.
        --curdir
        --ruby=/Users/u16suzu/.rvm/rubies/ruby-1.9.2-p290/bin/ruby
        --with-objclib
        --without-objclib
/Users/u16suzu/.rvm/rubies/ruby-1.9.2-p290/lib/ruby/1.9.1/mkmf.rb:368:in `try_do': The complier failed to generate an executable
 file. (RuntimeError)
You have to install development tools first.
        from /Users/u16suzu/.rvm/rubies/ruby-1.9.2-p290/lib/ruby/1.9.1/mkmf.rb:435:in `try_link0'
        from /Users/u16suzu/.rvm/rubies/ruby-1.9.2-p290/lib/ruby/1.9.1/mkmf.rb:440:in `try_link'
        from /Users/u16suzu/.rvm/rubies/ruby-1.9.2-p290/lib/ruby/1.9.1/mkmf.rb:552:in `try_func'
        from /Users/u16suzu/.rvm/rubies/ruby-1.9.2-p290/lib/ruby/1.9.1/mkmf.rb:748:in `block in have_library'
        from /Users/u16suzu/.rvm/rubies/ruby-1.9.2-p290/lib/ruby/1.9.1/mkmf.rb:693:in `block in checking_for'
        from /Users/u16suzu/.rvm/rubies/ruby-1.9.2-p290/lib/ruby/1.9.1/mkmf.rb:280:in `block (2 levels) in postpone'
        from /Users/u16suzu/.rvm/rubies/ruby-1.9.2-p290/lib/ruby/1.9.1/mkmf.rb:254:in `open'
        from /Users/u16suzu/.rvm/rubies/ruby-1.9.2-p290/lib/ruby/1.9.1/mkmf.rb:280:in `block in postpone'
        from /Users/u16suzu/.rvm/rubies/ruby-1.9.2-p290/lib/ruby/1.9.1/mkmf.rb:254:in `open'
        from /Users/u16suzu/.rvm/rubies/ruby-1.9.2-p290/lib/ruby/1.9.1/mkmf.rb:276:in `postpone'
        from /Users/u16suzu/.rvm/rubies/ruby-1.9.2-p290/lib/ruby/1.9.1/mkmf.rb:692:in `checking_for'
        from /Users/u16suzu/.rvm/rubies/ruby-1.9.2-p290/lib/ruby/1.9.1/mkmf.rb:743:in `have_library'
        from extconf.rb:10:in `<main>'

v8が複数入っていると思うので新しい方を消す
# gem uninstall libv8

gccのシンボリックリンクを貼る。
# sudo ln -s /usr/bin/gcc /usr/bin/gcc-4.2

参考になりました！
http://blog.calltella.com/
http://loveless-ainakimono.seesaa.net/article/232323709.html