我是 Ruby on Rails 的新手，安装时出现以下错误。
请注意，我已经安装了最新的 Xcode 并安装了命令行工具，并且我使用的是 OSX 10.8.4。我也安装了 Ruby 2.0.0。
-bash-3.2$ sudo gem install rails
Password:
Building native extensions. This could take a while...
ERROR: Error installing rails:
ERROR: Failed to build gem native extension.
/Users/WillieTran/.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/WillieTran/.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/WillieTran/.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/WillieTran/.rvm/rubies/ruby-2.0.0-p247/lib/ruby/2.0.0/mkmf.rb:519:in `try_link0'
from /Users/WillieTran/.rvm/rubies/ruby-2.0.0-p247/lib/ruby/2.0.0/mkmf.rb:817:in `try_run'
from extconf.rb:24:in `<main>'
Gem files will remain installed in /Users/WillieTran/.rvm/rubies/ruby-2.0.0-p247/lib/ruby/gems/2.0.0/gems/atomic-1.1.12 for inspection.
Results logged to /Users/WillieTran/.rvm/rubies/ruby-2.0.0-p247/lib/ruby/gems/2.0.0/gems/atomic-1.1.12/ext/gem_make.out