introduces a dummy output for local audio
Changes configure.ac to control the with-alsa and with-oss4 separately.
This makes it possible to only use the dummy output and compile without
alse+oss4.
In some complex ALSA setups, using PCM or Master is not the right thing
to do, so allow a custom mixer element name to be specified in the config
file for this use case.
Request and initial patch by Kurt Vanderlinden <kurt.vanderlinden@skynet.be>.