Clang on Windows targeting gcc requirements

I looked into this ~18 months ago when I discovered that BoostPP disables variadic macros under the VS 2013 preprocessor even though compiler v120 officially supports them.
The combination of having both MSC_VER and EDG defined was not correctly handled.
Given that EDG is an automatic predefine on this compiler I guessed Microsoft had switched to EDG. They won’t confirm it of course. The closest I could get was confirmation from MS that this compiler does have a new preprocessor front end and from EDG that they are supplying CPP front ends to Microsoft.
The end result is the same. I’m not sure that the VS CPP is as broken as it appears. Other than this bug I’ve had no trouble with BoostPP under MinGW clang or Visual Studio.