Dear Maintainer,

operator== and operator!= at the end of guiddef.h lack extern "C++".
This will cause compiler errors if any other operator== in the project
lacks the extern "C++" tag.

This is an upstream bug, but mingw's bug tracker is down,
so the best I can do is file it to a downstream.

Wine also has the same bug, I've submitted a patch to them.

https://gitlab.winehq.org/wine/wine/-/merge_requests/8089