SDL: Add library paths of all SDL sub-packages to sdl-config

Because of how nixpkgs works, all the SDL-umbrella libraries are in
different folders. On a usual system, when using `sdl-config --libs`,
this means that SDL_gfx would also be on the linker path. Some packages
seem to depend on this being the case.

pkgs/development/libraries/SDL/setup-hook.sh

@@ -1,6 +1,7 @@
 addSDLPath () {
   if [ -e "$1/include/SDL" ]; then
     export SDL_PATH="$SDL_PATH $1/include/SDL"
+    export SDL_LIB_PATH="$SDL_LIB_PATH -L$1/lib"
   fi
 }