title: gem install mysql2 fails
body: <p>I got this error:</p>
  
  <pre><code>sudo gem install mysql2
  Building native extensions.  This could take a while...
  ERROR:  Error installing mysql2:
      ERROR: Failed to build gem native extension.
  
  /usr/bin/ruby1.8 extconf.rb
  checking for rb_thread_blocking_region()... no
  checking for mysql.h... no
  checking for mysql/mysql.h... no
  -----
  mysql.h is missing.  please check your installation of mysql and try again.
  -----
  *** 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=/usr/bin/ruby1.8
      --with-mysql-config
      --without-mysql-config
  
  
  Gem files will remain installed in /var/lib/gems/1.8/gems/mysql2-0.2.6 for inspection.
  Results logged to /var/lib/gems/1.8/gems/mysql2-0.2.6/ext/mysql2/gem_make.out
  </code></pre>
  
  <p>I have tried:</p>
  
  <ul>
  <li>uninstall mysql2 and install it again</li>
  <li>uninstall mysql with apt-get</li>
  <li>installd dev-tools from mysql</li>
  <li>and almost all the pages on Google</li>
  </ul>
  
  <p>How can I fix this?</p>