Freeciv

sdl2-client flatpak build depends on SDL2_gfx, which is built as a module. On some targets build of that module fails because of the ancient build system (e.g. config.guess / config.sub) of SDL2_gfx project.

The solution is to implement build ourselves, instead of relying on automatic build system detection of flatpak.

Only S3_0 and S3_1 affected, as later branches build from included SDL2_gfx copy.