Ubuntu 16.04 LTS で mysql2 gem がインストールできない場合

結論

libmysqlclient-dev を入れましょう。

# apt-get install -y libmysqlclient-dev

# gem install mysql2
Building native extensions.  This could take a while...
Successfully installed mysql2-0.4.5
Parsing documentation for mysql2-0.4.5
Installing ri documentation for mysql2-0.4.5
Done installing documentation for mysql2 after 1 seconds
1 gem installed