title: I can't run "bundle update" because of "mysql2" gem
body: <p>I have this in the Gemfile:</p>
  
  <pre><code>gem 'mysql2'
  </code></pre>
  
  <p>But when I run <strong>bundle update</strong>, I get this error message:</p>
  
  <pre><code>An error occurred while installing mysql2 (0.3.16), and Bundler cannot continue.
  Make sure that `gem install mysql2 -v '0.3.16'` succeeds before bundling.
  </code></pre>
  
  <p>I've tried to move this into the production section, like this:</p>
  
  <pre><code>group :production do
    gem 'mysql2'
  end
  </code></pre>
  
  <p>But after running <strong>bundle update</strong>, the result is the same. This section is processed only in the production mode, or not?</p>
  
  <p>How to get rid of this error message on localhost?</p>
  
  <p><strong>EDIT: The whole error message:</strong></p>
  
  <pre><code>Gem::Installer::ExtensionBuildError: ERROR: Failed to build gem native extension.
  
          /Users/radek/.rvm/rubies/ruby-1.9.3-p385/bin/ruby extconf.rb
  checking for ruby/thread.h... *** 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
      --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/radek/.rvm/rubies/ruby-1.9.3-p385/bin/ruby
  /Users/radek/.rvm/rubies/ruby-1.9.3-p385/lib/ruby/1.9.1/mkmf.rb:381:in `try_do': The compiler failed to generate an executable file. (RuntimeError)
  You have to install development tools first.
      from /Users/radek/.rvm/rubies/ruby-1.9.3-p385/lib/ruby/1.9.1/mkmf.rb:506:in `try_cpp'
      from /Users/radek/.rvm/rubies/ruby-1.9.3-p385/lib/ruby/1.9.1/mkmf.rb:931:in `block in have_header'
      from /Users/radek/.rvm/rubies/ruby-1.9.3-p385/lib/ruby/1.9.1/mkmf.rb:790:in `block in checking_for'
      from /Users/radek/.rvm/rubies/ruby-1.9.3-p385/lib/ruby/1.9.1/mkmf.rb:284:in `block (2 levels) in postpone'
      from /Users/radek/.rvm/rubies/ruby-1.9.3-p385/lib/ruby/1.9.1/mkmf.rb:254:in `open'
      from /Users/radek/.rvm/rubies/ruby-1.9.3-p385/lib/ruby/1.9.1/mkmf.rb:284:in `block in postpone'
      from /Users/radek/.rvm/rubies/ruby-1.9.3-p385/lib/ruby/1.9.1/mkmf.rb:254:in `open'
      from /Users/radek/.rvm/rubies/ruby-1.9.3-p385/lib/ruby/1.9.1/mkmf.rb:280:in `postpone'
      from /Users/radek/.rvm/rubies/ruby-1.9.3-p385/lib/ruby/1.9.1/mkmf.rb:789:in `checking_for'
      from /Users/radek/.rvm/rubies/ruby-1.9.3-p385/lib/ruby/1.9.1/mkmf.rb:930:in `have_header'
      from extconf.rb:9:in `&lt;main&gt;'
  
  
  Gem files will remain installed in /Users/radek/.rvm/gems/ruby-1.9.3-p385/gems/mysql2-0.3.16 for inspection.
  Results logged to /Users/radek/.rvm/gems/ruby-1.9.3-p385/gems/mysql2-0.3.16/ext/mysql2/gem_make.out
  An error occurred while installing mysql2 (0.3.16), and Bundler cannot continue.
  Make sure that `gem install mysql2 -v '0.3.16'` succeeds before bundling.
  </code></pre>
  
  <p>Thank you</p>