Do we even want to make this configurable at the level of vkd3d-compiler?
Ultimately I think we do, but this probably isn't the way. What we want is probably some way to specify a target GPU or feature set, somewhat along the lines of gcc's "-march" option. These would then imply a particular set of supported extensions and features. Similarly, we could imagine vkd3d-compiler querying the features of the host GPU, along the lines of "-march=native".
Building from there, we could probably add options to enable/disable individual extensions and features as well.