[PATCH] Allow _MSC_EXTENSIONS macro on non-Windows targets

[Robinson, Paul]

>   I think it would be better to use `__has_extension` instead:
> http://clang.llvm.org/docs/LanguageExtensions.html#has-feature-and-has-
> extension

We're trying to arrange that __wchar_t is always available, but
if the compiler predefines it then we can't.  What __has_extension
test can we make that tells us when this name is predefined?
Skimming through the doc it didn't look like there was one for this.
Thanks,
--paulr