widl: Added default __midl, _WIN32 and _WIN64 defines.

[Alexandre Julliard]

Jacek Caban <jacek at codeweavers.com> writes:

> They are defined by default by midl.

__midl is supposed to be set to the midl compiler version, and I'm not
entirely convinced it's appropriate for us to define it.

Also, _WIN64 can't depend on how widl was built, since it can generate
both 32-bit and 64-bit code. You'd have to parse everything twice if you
wanted to make use of this.

-- 
Alexandre Julliard
julliard at winehq.org