On Linux, due to the way we include SDL2 as a submodule, it makes it
difficult for us to specify which SDL_config.h we intended to include.
Before, CMake would default to the dummy one included with SDL and
ignore the generated one.
This tells CMake to use the generated one. In addition, we define
USING_GENERATED_CONFIG_H to throw an error in case the dummy config is
used by accident. Fixes Vulkan not working on Linux yuzu-cmd.
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