CONFIGURE: Add missing SDL_BACKEND define for the OpenPandora backend.

* Not really sure when/how it got lost but as it's lack  spectacularly
  breaks the backend it is a really good idea to put it back ;)

Thanks to Max for the heads up.

configure

@@ -3083,6 +3083,7 @@ case $_backend in
 		INCLUDES="$INCLUDES `$_sdlconfig --prefix="$_sdlpath" --cflags`"
 		LIBS="$LIBS `$_sdlconfig --prefix="$_sdlpath" --libs`"
 		LDFLAGS="$LDFLAGS"
+		DEFINES="$DEFINES -DSDL_BACKEND"
 		;;
 	ps2)
 		# TODO ps2