How much code breaks without this? Do we really want to do this? My inclination is to get by without it. In particular, libyuv has made changes to not assume that x86_64 implies GNUC functionality.
This condition is incorrect, Microsoft builds apps such as Office for Mac with clang with -fms-extensions. They very likely include system headers that rely on x86_64 &co. This should probably be:
if (MicrosoftExt) ... _M_* macros if (MSVCCompat) return; ...
In our code base I think it's only occurrence of this problem, but I do
expect this to be the sort of thing that breaks more than just libyuv in
the wild. That being said, I'll leave it up to you whether we should take
this or not, and will address the comment below when landing if we decide
to to do so.