Why not just standalone?
I don't think that's necessarily clear enough because technically vkd3d native, etc is standalone. You can use it without anything.
This makes it simpler as it just says enable_d3d12 which implies wouldn't be built otherwise -- which is true.
How about defaulting this option to True if target machine is Windows instead?
Yes. I agree on that. There is very little reason to not build the standalone dll when targeting Windows.
Nothing extracted yet.
I always forget whether its enable_d3d12_standalone or enable_standalone_d3d12 -- we should probably just make this enable_d3d12.