The runtimes use *_STANDALONE_BUILD=OFF to signify that clang is an in-tree target. This is not the case with the runtime builds, so we really need this set to ON.
In order to resolve the issues phosek was having with checks, we should use checks that don't link. We can use compiler-rt's try_compile_only as a basis for that.
This patch is *required* to be able to run the runtime libraries check-* targets.