This patch implements a Genode-specific audio backend for SDL.
The audio volume (in percent) can be configured in the config file of the
SDL application:
<config>
<sdl_audio_volume value="100"/>
</config>
Fixes#204.
This patch enables the use of threads and locking mechanisms in SDL
applications. The 'pthread' libary is used as backend. Not all features
are currently supported.
Fixes#185.