title: failed gem install rails on mac ERROR: Failed to build gem native extension
body: <p>I downloaded xcode, command line tools, home brew, rvm, and ruby, but when I use the command <code>sudo gem install rails</code> I get an error message. Not sure what to do any help would be great, I want to install rails on my new mac.</p>
  
  <pre><code>ERROR:  While executing gem ... (Errno::EACCES)
      Permission denied - /Users/cheatermoves/.rvm/gems/ruby-2.0.0-p247/gems/atomic-1.1.14/test/test_atomic.rb
  marshalls-MacBook-Air:~ cheatermoves$ sudo gem install rails
  Building native extensions.  This could take a while...
  ERROR:  Error installing rails:
      ERROR: Failed to build gem native extension.
  
      /Users/cheatermoves/.rvm/rubies/ruby-2.0.0-p247/bin/ruby extconf.rb
  *** 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/cheatermoves/.rvm/rubies/ruby-2.0.0-p247/bin/ruby
      --with-atomic_reference-dir
      --without-atomic_reference-dir
      --with-atomic_reference-include
      --without-atomic_reference-include=${atomic_reference-dir}/include
      --with-atomic_reference-lib
      --without-atomic_reference-lib=${atomic_reference-dir}/
  /Users/cheatermoves/.rvm/rubies/ruby-2.0.0-p247/lib/ruby/2.0.0/mkmf.rb:434:in `try_do': The compiler failed to generate an executable file. (RuntimeError)
  You have to install development tools first.
      from /Users/cheatermoves/.rvm/rubies/ruby-2.0.0-p247/lib/ruby/2.0.0/mkmf.rb:519:in `try_link0'
      from /Users/cheatermoves/.rvm/rubies/ruby-2.0.0-p247/lib/ruby/2.0.0/mkmf.rb:817:in `try_run'
      from extconf.rb:26:in `&lt;main&gt;'
  </code></pre>