Guard unsafeFlags with Windows-only conditional to restore SwiftPM compatibility (#239)#240

QuietMisdreavus merged 1 commit intoswiftlang:mainswiftlang/swift-markdown:mainfrom

dlskawns96:fix/guard-unsafeflags-nonwindowsdlskawns96/swift-markdown:fix/guard-unsafeflags-nonwindowsCopy head branch name to clipboard