OSS: define HAVE_OSS, and disable on Linux by default
In many cases, OSS was used when the builder forgot to install the alsa-lib development headers. This should not be an issue anymore, as ALSA is now required on Linux by default. I believe the fallback to OSS from ALSA and PulseAudio is causing more confusion than it solves problem. Besides, the OSS emulation by ALSA cannot mix, so OSS is almost completely useless on Linux, unless OSSv4 was specifically installed. Moreover, Linux distributions have started dropping OSS completely.
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