Mac OS X Mavericks で bundle install した時 nokogiri が "libiconv is missing."でインストールできなかった

##問題
railsチュートリアルをやっておりましたところ、bundle install で nokogiri というgemがインストール出来ない問題に出くわしました。

    Gem::Installer::ExtensionBuildError: ERROR: Failed to build gem native extension.

        /Users/niwatako/.rbenv/versions/2.0.0-p481/bin/ruby extconf.rb --with-xml2-include=/usr/local/opt/libxml2/include/libxml2 --with-xml2-lib=/usr/local/opt/libxml2/lib --with-xslt-dir=/usr/local/opt/libxslt
    Building nokogiri using packaged libraries.
    -----
    libiconv is missing.  please visit http://nokogiri.org/tutorials/installing_nokogiri.html for help with installing dependencies.
    -----
    *** 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/niwatako/.rbenv/versions/2.0.0-p481/bin/ruby
    	--help
    	--clean
    	--use-system-libraries
    	--enable-static
    	--disable-static
    	--with-zlib-dir
    	--without-zlib-dir
    	--with-zlib-include
    	--without-zlib-include=${zlib-dir}/include
    	--with-zlib-lib
    	--without-zlib-lib=${zlib-dir}/lib
    	--enable-cross-build
    	--disable-cross-build


    Gem files will remain installed in /Users/niwatako/project/rails/4.0.5/sample_app/vendor/bundle/gems/nokogiri-1.6.3.1 for inspection.
    Results logged to /Users/niwatako/project/rails/4.0.5/sample_app/vendor/bundle/gems/nokogiri-1.6.3.1/ext/nokogiri/gem_make.out
    An error occurred while installing nokogiri (1.6.3.1), and Bundler cannot
    continue.
    Make sure that `gem install nokogiri -v '1.6.3.1'` succeeds before bundling.

## この時のGemfile

    source 'https://rubygems.org'
    ruby '2.0.0'
    #ruby-gemset=railstutorial_rails_4_0

    gem 'rails', '4.0.5'

    group :development, :test do
      gem 'sqlite3', '1.3.8'
      gem 'rspec-rails', '2.13.1'
      gem 'guard-rspec', '2.5.0'
      gem 'spork-rails', '4.0.0'
      gem 'guard-spork', '1.5.0'
      gem 'childprocess', '0.3.6'
    end

    group :test do
      gem 'selenium-webdriver', '2.35.1'
      gem 'capybara', '2.1.0'

      # Uncomment this line on OS X.
      # gem 'growl', '1.0.3'

      # Uncomment these lines on Linux.
      # gem 'libnotify', '0.8.0'

      # Uncomment these lines on Windows.
      # gem 'rb-notifu', '0.0.4'
      # gem 'win32console', '1.3.2'
    end

    gem 'sass-rails', '4.0.2'
    gem 'uglifier', '2.1.1'
    gem 'coffee-rails', '4.0.1'
    gem 'jquery-rails', '3.0.4'
    gem 'turbolinks', '1.1.1'
    gem 'jbuilder', '1.0.2'

    group :doc do
      gem 'sdoc', '0.3.20', require: false
    end

    group :production do
      gem 'pg', '0.15.1'
      gem 'rails_12factor', '0.0.2'
    end

## 解決
これを解決するには libiconv を homebrew で入れてやって、bundler に nokogiri 用のビルド設定をしてやります。

    # homebrew公式に libiconv が無いので libiconv formula を追加
    brew tap homebrew/dupes
    
    # libxml2、libxslt、libiconv のインストール
    brew install libxml2 libxslt libiconv
    brew link --force libxml2 libxslt libiconv
    
    # nokogiri 用のビルド設定をbundlerにいれてやる
    bundle config build.nokogiri --use-system-libraries
    
    # 無事インストールできるはず
    bundle install

ちなみに`bundle config build.nokogiri --use-system-libraries`の設定がどこに保存されるかというと、`~/.bundle/config`に保存されています。

    ---
    BUNDLE_BUILD__NOKOGIRI: --use-system-libraries