Add SPM support to par2cmdline-turbo

[Coeur]

This is a followup to Parchive#222.

This adds Swift Package Manager support on various platforms (macOS, Linux, Android, Windows), which can easily be verified on macOS by double-clicking on the Package.swift.

The first commit is for par2cmdline. The second commit is for par2cmdline-turbo.

[animetosho]

Thanks, I'll look into this if it gets accepted upstream.

If this is effectively another build tool, can it perform compiler checks like this and pass options if they're available?

[Coeur]

Currently such compiler checks aren't supported by SPM. It seems to be by design.
I could be wrong but I think they want builds to be universal, and when hardware features are needed, it would be a runtime check instead of a build thing.
Or they assume it's the compiler job to auto-detect what flags it needs by default.

[animetosho]

Thanks for the info.

The builds are 'universal' and features are runtime checked. You need to enable them in the compiler however, otherwise the feature won't be compiled in at all.

If SPM doesn't support some way to check/enable flags on specific files, it's not so workable as the fast kernels in this project rely on those flags being enabled.