When the detected SDK for CMAKE_OSX_SYSROOT and friends has the same
version as your current macOS system version, CMake helpfully doesn't
set CMAKE_OSX_DEPLOYMENT_TARGET. Unfortunately, in this case, swiftc
will default to macOS 10.4, which is absolutely ancient. Grab the target
triple from the -print-target-info JSON when CMAKE_OSX_DEPLOYMENT_TARGET
is not provided at configure time.