sdl2-config FindSDL2.cmake does not handle a change in Debian's SDL2 directory layout (there is a bug against libsdl2 for that, https://bugs.debian.org/951087). SDL2 >= 2.0.4 has a sdl2-config.cmake that eliminates the need for FindSDL2.cmake.
Try to fix #993132, cherry-picking upstream patches https://github.com/fifengine/fifengine/pull/1076 and https://github.com/fifengine/fifengine/commit/f37c31c6cea74d50f1d635d876253a7b1f64630b