Guard unsafeFlags with Windows-only conditional to restore SwiftPM compatibility (#239)

[dlskawns96]

Bug/issue #239

Summary

This PR addresses the SwiftPM resolution failure introduced in version 0.7.0 where the Markdown target included unsafeFlags in its swiftSettings.

On non-Windows platforms (iOS, macOS, Linux), SwiftPM forbids depending on packages that contain unsafe build flags as tagged dependencies, leading to errors like: the target ‘Markdown’ in product ‘Markdown’ contains unsafe build flags

Implementation overview

• Refactored swiftSettings so that unsafeFlags are only included when building on Windows.
• On non-Windows, the manifest no longer includes unsafeFlags, restoring SwiftPM compatibility.

Dependencies

None.
(Future work may involve updating swift-cmark to move the CMARK_GFM_STATIC_DEFINE into its cSettings.)

Testing

Steps:

1. Add this branch as a dependency in a SwiftPM project (iOS/macOS).

   .package(url: "https://github.com/dlskawns96/swift-markdown.git",
            .branch("fix/guard-unsafeflags-nonwindows"))

2. Run xcodebuild -resolvePackageDependencies or open in Xcode.
3. Confirm the package resolves successfully (previously failed with unsafe flags error).
4. On Windows, verify that the build still succeeds with the required CMARK_GFM_STATIC_DEFINE.

Checklist

Make sure you check off the following items. If they cannot be completed, provide a reason.

• Added tests (not applicable, this is a manifest-only change)
• Ran the ./bin/test script and it succeeded
• Updated documentation if necessary

[QuietMisdreavus]

Is this issue happening with the just-tagged 0.7.0 release? If so, i can cherry-pick it and release a 0.7.1 tag with this fix.

[QuietMisdreavus]

@swift-ci Please test

[QuietMisdreavus]

Oh, i just saw the issue you filed. I can go ahead and publish a 0.7.1 tag when CI passes and i can merge this PR.

[QuietMisdreavus]

Thanks so much for opening the PR! I'll let you know when i can publish a new tag with the fix in it.

[dlskawns96]

Thanks for the quick review and merge! 🙏

[compnerd]

This doesn't work, this restricts to building on windows, not for windows.