Make OpenMP optional instead of default dependency

[solaoi]

Make OpenMP optional to improve compatibility with MacOS

Background

Currently, OpenMP is specified as a default dependency in Cargo.toml. However, OpenMP is not installed by default on MacOS systems, which can cause unexpected build failures, especially in CI environments like GitHub Actions macOS runners.

Changes

• Remove OpenMP from default features
• Make it available as an optional dependency, similar to whisper.cpp's approach

Benefits

• Better out-of-the-box experience on MacOS
• No additional libomp installation required for basic builds
• Improved CI/CD workflow, especially on macOS runners
• More flexible dependency management for users

Note

Users who want to use OpenMP can still enable it explicitly through features.

[tazz4843]

Huh never even realized OpenMP was enabled by default now, not sure when that happened. Thanks for the PR :)

[arizhih]

Huh never even realized OpenMP was enabled by default now, not sure when that happened. Thanks for the PR :)

Sorry for late reply, i did it in #169 because it’s enabled by default in whisper.cpp too. But maybe it’s better not to enable it by default here :)