Build fails with libsdl2 2.0.20

[Mikolaj]

This

  pkgconfig-depends:
    sdl2 >= 2.0.6

seems inaccurate. It requires sdl2 >= 2.0.10 according to #240 and it seems to fail with 2.0.20 as seen in https://ci.appveyor.com/project/Mikolaj/allure/builds/42817009

Of course, ideally it'd work with all sdl2 versions, perhaps using cabal flags to juggle compatibility settings.

[dpwiz]

Everything is quiet and building for some time.