R1:3f1a78450cce

Require mysql or mysqli, not both

Summary: We use mysqli if it's available by default. Don't require installs to build with mysql.

Test Plan: Applied to new secure.phabricator.com install.

Reviewers: chad, vrana

Reviewed By: chad

CC: aran

Differential Revision: https://secure.phabricator.com/D5194