Log Message:
use mk/mysql.buildlink3.mk instead of databases/mysql-client/buildlink3.mk,
so that we'd not force dependance on specific MySQL version, and instead pick
the currently installed mysql*-client (or install the default if there
is no mysql-client package installed yet)

this makes package buildable with arbitrary MySQL version, such as 3.23.x,
4.0.x or 4.1.x