sdl2: Tweaked Makefile

Work around configure picking up host X11 headers by disabling X11
support explicitly. And try to avoid similar issues by disabling
not wanted features instead of relying on the autodetect to not
enable them.

Use the --disable-rpath option instead of patching sdl2-config.
opendingux: GCW0: defconfig: Enable screen

This is useful to start a program for testing, disconnect the network
and check in on it later. In some cases "nohup" is sufficient, but
that doesn't support interaction.