Make Linux CPU builds runnable

[zheng1]

Summary

• define _GNU_SOURCE for Linux builds so POSIX/GNU APIs used by the code are declared under -std=c99
• default the CLI to the CPU reference backend when built without Metal support
• keep non-Metal server builds explicit by returning a clear unsupported-server message
• allow download_model.sh to place large GGUF files outside the repository via DS4_GGUF_DIR

Testing

• make -j 16
• make test
• ./ds4 --help
• ./ds4-server --port 18000

Notes

This does not add a production non-Metal backend. The server path still depends on Metal sessions; the Linux path is limited to CPU reference/debug CLI use and model inspection.