mysql-connector-c++: Add MariaDB & Percona flags

Hi-
Just added some flags for so users can choose between Oracle MySQL, MariaDB, and Percona. MySQL is still default. (This code is nearly exactly the same as [the hhvm formula](https://github.com/mcuadros/homebrew-hhvm/blob/master/Formula/hhvm.rb).)
Thank you for all the hard work!
MM