Replace SDK requirements and make it linux native

[blinkysc]

Probably a very big ask and not something that would happen any time soon. But... Would love to see modern API's used that can just be compiled cross platform =D

[JohnsterID]

This would be the potential starting point: #486
Then I'm guessing SDL, OpenAL Soft, ffmpeg similar to the Thyme plan.
Quite a bit listed before all that though: https://github.com/TheSuperHackers/GeneralsGameCode/milestone/3

[JohnsterID]

Some links if others come across this topic.
ffmpeg: #714
OpenAL Soft,: #784
SDL3: https://github.com/Fighter19/CnC_Generals_Zero_Hour/commits/develop/
DX9: #1575