jextract: Evaluate IfConfigDecl and add --static-build-config option

[sidepelican]

• jextract: Evaluate IfConfigDecl for simple patterns #633

It seems difficult for the jextract plugin to reliably execute -print-static-build-config.
Instead, this PR embeds a fixed JExtractDefaultBuildConfiguration.

The policy for this fixed configuration is to resolve the widest possible range of #if conditions.

This approach introduces a risk where jextract might generate code for functions that are not actually present in the loaded Swift library, which would result in a runtime error.
However, I believe providing access to these functions by default is more useful than having them be completely unavailable.

To support cases where a specific configuration is required, I have added a --static-build-config <filepath> option to jextract.
This allows users to provide their own generated JSON file to customize the BuildConfiguration.

[ktoso]

nice, that's what I would expect here

[ktoso]

Looks great, minor concerns about the big features json... I'd rather we don't have to commit that. We may have to NOT have a .shared at all and just compute this during generation instead

[sidepelican]

@ktoso The new BuildToolsPlugin now allows running -print-static-build-config during builds.

It uses a global swift command so the results may not always match the exact build-time swift version.

[sidepelican]

Is the Android CI having some trouble at the moment?
Also, since 6.3 has been officially released, I thought it would be a good idea to add a CI job pinned specifically to 6.3.

[ktoso]

Yeah we should do a 6.3 job, I'll follow up on it

[madsodgaard]

Great addition! Could we perhaps add some docs about this feature?

[ktoso]

Docs would be good about this, it's worth understanding how this works.

I'm offline this evening already, can merge in the morning