Conditionalize Windows flags to mitigate SwiftPM crash (#683)
Conditionalize Windows flags to mitigate SwiftPM crash (#683)

SwiftPM 5.3 and earlier will crash when an unknown platform is used in a
conditional. To avoid the crash, condtionalize the flags for now. The
SwiftPM bug has been fixed and once it is available in a released
version, we can removed this workaround.
apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild