SDL2: pass explicit --without-x flag to configure

Fixes build on Darwin for me, possibly necessary on other systems as
well

pkgs/development/libraries/SDL2/default.nix

···
         --disable-oss --disable-x11-shared
         --disable-pulseaudio-shared --disable-alsa-shared
         ${if alsaSupport then "--with-alsa-prefix=${attrs.alsaLib}/lib" else ""}
+        ${if (!x11Support) then "--without-x" else ""}
       '';
 in
 stdenv.mkDerivation rec {