games/alephone: Spell FFMPEG_* correctly

Disable FFMPEG option by default and mark it as broken since it does not build:

SDL_ffmpeg.c:89:103: warning: declaration of 'enum PixelFormat' will not be visible outside of this function [-Wvisibility]
struct SwsContext* getContext( SDL_ffmpegConversionContext **context, int inWidth, int inHeight, enum PixelFormat inFormat, int outWidth, int outHeight, enum PixelFormat outFormat )

                      ^

SDL_ffmpeg.c:89:115: error: variable has incomplete type 'enum PixelFormat'
struct SwsContext* getContext( SDL_ffmpegConversionContext **context, int inWidth, int inHeight, enum PixelFormat inFormat, int outWidth, int outHeight, enum PixelFormat outFormat )

                                  ^