There's a common problem when you run bundle install and it tries to install the msql2 gem but fails when mysql itself is not installed on the machine. What if you have mysql installed on a different machine (because you don't want your web application which is accessible to the public and your mysql database on the same machine)? In this case, do you still have to install mysql on the web application machine just for the sole purpose of making this error go away?
You have to install the MySQL **client** command-line software on your Rails server, so the mysql2 gem will build clean against it. Be sure to add the dev headers, too.
Walter