Compilation errors for clang-cl in VS22

[Caball009]

(TheSuperHackers/GeneralsGameCode#888 (comment))

I noticed that the gt2 project currently cannot be compiled with clang-cl in Visual Studio 22. There seems to be an issue around the macro va_arg which gets a different definition with clang-cl: https://github.com/TheAssemblyArmada/GamespySDK/blob/master/src/gt2/gt2encode.c#L66-L140