Clang on Windows targeting gcc requirements

It’s quite possible that it’s a bug on the MS side, although a strange one to have EDG predefined in the main preprocessor if it’s not the same as the intellisense one.
It would be useful to know if the definition is still there in the VS2015 toolchain.
It has been too long since I dug into this and I have other things cooking at the moment so probably best if I leave it in Edward’s capable hands.