macOS has no OSS (/dev/dsp) support, but dsd-fme currently compiles and links OSS code unconditionally, leading to build and link failures.
This PR:
* disables OSS backend cleanly on macOS
* provides a no-op openOSSOutput() stub to satisfy callers
* guards OSS-specific headers and ioctl usage
* preserves existing behavior on Linux/OSS platforms
This allows dsd-fme to build and run correctly on macOS using PulseAudio/CoreAudio paths.
83d7460a0b