Port CoreAudio code to AVFoundation (OS X) for Extended

[THLeaderH]

This patch was incorporated in standard but not yet in extended. This is required to compile the code on the latest OS X.
This patch enables to use AV Foundation to play sound instead of QTKit, with which the latest Mac cannot compile the code.
original: https://forum.simutrans.com/index.php?topic=16675.0
The content is same as that of standard. I threw a pull request to James' repository. https://github.com/jamespetts/simutrans-extended/pull/84

[jamespetts]

Incorporated - thank you.