Building without SDL currently breaks

[mid-kid]

System info:
Gentoo 17.1 amd64
libsdl and libsdl2 both installed in system.

Steps to reproduce:

mkdir build
cd build
cmake -DBUILD_SDL=OFF ..
make

/home/mid-kid/mgba/src/platform/qt/InputController.cpp: In member function 'void QGBA::InputController::setPreferredGamepad(uint32_t, int)':
/home/mid-kid/mgba/src/platform/qt/InputController.cpp:275:76: error: 's_sdlEvents' was not declared in this scope
  275 | L_JoystickGetGUIDString(SDL_JoystickGetGUID(SDL_JoystickListGetPointer(&s_sdlEvent .joysticks, index)->joystick), name, sizeof(name));
      |                                                                         ^~~~~~~~~~

/home/mid-kid/mgba/src/platform/qt/InputController.cpp:275:48: error: 'SDL_JoystickListGetPointer' was not declared in this scope
  275 |  SDL_JoystickGetGUIDString(SDL_JoystickGetGUID(SDL_JoystickListGetPointer(&s_sdlEvents.joysticks, index)->joystick), name, sizeof(name));
      |                                                ^~~~~~~~~~~~~~~~~~~~~~~~~~
/home/mid-kid/mgba/src/platform/qt/InputController.cpp:275:28: error: 'SDL_JoystickGetGUID' was not declared in this scope
  275 |  SDL_JoystickGetGUIDString(SDL_JoystickGetGUID(SDL_JoystickListGetPointer(&s_sdlEvents.joysticks, index)->joystick), name, sizeof(name));
      |                            ^~~~~~~~~~~~~~~~~~~
/home/mid-kid/mgba/src/platform/qt/InputController.cpp:275:2: error: 'SDL_JoystickGetGUIDString' was not declared in this scope
  275 |  SDL_JoystickGetGUIDString(SDL_JoystickGetGUID(SDL_JoystickListGetPointer(&s_sdlEvents.joysticks, index)->joystick), name, sizeof(name));
      |  ^~~~~~~~~~~~~~~~~~~~~~~~~
make[2]: *** [qt/CMakeFiles/mgba-qt.dir/build.make:584: qt/CMakeFiles/mgba-qt.dir/InputController.cpp.o] Error 1
make[1]: *** [CMakeFiles/Makefile2:380: qt/CMakeFiles/mgba-qt.dir/all] Error 2
make: *** [Makefile:152: all] Error 2

I think there's two options here: Either drop the SDL dependency for the Qt frontend, or don't allow BUILD_QT==ON && BUILD_SDL==OFF.

[lotharsm]

I can confirm this for msys2/mingw-w64.