Fix compatibility with modern MySQL

Summary:
MySQL 8.0 onward changed a few details in the client API, and embedded
support has apparently been removed completely. This PR makes
minimally invasive changes that fix the related compile errors in a
(hopefully) backward compatible manner.

Test Plan:
compile against MySQL <= 5, MySQL >= 8, and various versions of MariaDB
(though it seems they didn't touch that part of the API yet)

Reviewers: asturmlechner, heikobecker

Reviewed By: asturmlechner, heikobecker

Subscribers: heikobecker, jfowkes, asturmlechner, wbauer, amarok-devel

Tags: Amarok

Differential Revision: https://phabricator.kde.org/D28165