gcc-7 allows `constexpr auto X = std::initializer_list<unsigned>{A1, A2,
...};`. gcc-8 rejects it:
```
common/arch/sdl/key.cpp:583:105: error: 'const std::initializer_list<const SDL_Scancode>{((const SDL_Scancode*)(&<anonymous>)), 3}' is not a constant expression
constexpr auto sticky_keys = {SDL_SCANCODE_CAPSLOCK, SDL_SCANCODE_SCROLLLOCK, SDL_SCANCODE_NUMLOCKCLEAR};
```
Switch to a macro and a fully anonymous list, which is accepted by both
versions.
Fixes: f491059ed7 ("Enable building with SDL2")
c65020bf04