Windows has several variations of runtime libraries: MT, MTd, MD, MDd depending on if you're linking it statically or dynamically, and for release or debug.
It looks like swiftc supports all of these options and uses MD by default (Proper Support for MSVCRT linkage) and when invoking swiftc directly. This seems to work just fine.
But I can't figure out how to specify which runtime to use when trying to build a swift library for windows using SwiftPM--it only ever wants to use MD. This is where it looks like SwiftPM is setting the option: here and here. Given that I tried changing the value in SDKSettings.plist -- and this looks like it works. However it's not a great experience to need to manually change this file to switch between debug and release builds.
I also tried just manually passing the -libc flag to SwiftPM, but then I get the default MD flag as well as the manually specified one.
Did I miss something that makes it possible to configure this more easily from SwiftPM?