As this has never worked right out of the box (the necessary files are in places not expected by the gem installer), here is what I do:
sudo gem install mysql -- --with-mysql-config=/usr/lib/mysql/mysql_config
Typography is the craft of endowing human language with a durable visual form, and thus with an independent existence. (...) Typography remains a source of true delight, true knowledge, true surprise.
Robert Bringhurst, The Elements of Typographic Style
As this has never worked right out of the box (the necessary files are in places not expected by the gem installer), here is what I do:
sudo gem install mysql -- --with-mysql-config=/usr/lib/mysql/mysql_config