[Linux] Define _GNU_SOURCE when building TSCclibc

[bnbarham]

Fixes missing declaration for posix_spawn_file_actions_addchdir_np on
Linux.

[bnbarham]

@swift-ci please test

[compnerd]

This matches what I had experienced but failed to upstream; thank you for taking care of it.

[tomerd]

thanks @bnbarham can you provide context to the fix? is there a test that is missing that could have prevented the omission?

[bnbarham]

thanks @bnbarham can you provide context to the fix? is there a test that is missing that could have prevented the omission?

It's currently failing on rebranch (corresponding to LLVM stable/20220421) - https://ci.swift.org/view/Swift%20rebranch/job/oss-swift-rebranch-pr-test-ubuntu-20_04/315. I didn't look too hard into what changed there, any ideas @compnerd?

An alternative to this change would be to update the definition of SPM_posix_spawn_file_actions_addchdir_np itself. It is already checking for GLIBC and minimum 2.29, so I assume if it doesn't exist any more that there might be a portable (ie. minus _np) now?

[tomerd]

@swift-ci smoke test

[compnerd]

I don't remember what exposed this :(. This is the correct thing though - there's no guarantee that the interfaces are available - they are inherently non-portable and require opt-in. I think that it might be that the importer doesn't define it now to correct other interactions?