move SDL_SavePNG to the wesnoth_sdl lib target

This is to possibly fix issue shadowm reported, and keep all the
things that SDL depends on together.

src/SConscript

@@ -162,6 +162,10 @@ libwesnoth_sdl_sources = Split("""
     tracer.cpp
     xBRZ/xbrz.cpp
     """)
+
+if env["png"]:
+    libwesnoth_sdl_sources.append("SDL_SavePNG/savepng.cpp")
+
 libwesnoth_sdl = client_env.Library("wesnoth_sdl", libwesnoth_sdl_sources)
 
 libcutter_sources = Split("""
@@ -575,9 +579,6 @@ if env["PLATFORM"] == "win32":
 if env["notifications"]:
     wesnoth_sources.append("desktop/dbus_notification.cpp")
 
-if env["png"]:
-    wesnoth_sources.append("SDL_SavePNG/savepng.cpp")
-
 wesnoth_sources.extend(client_env.Object("game_preferences_display.cpp", EXTRA_DEFINE = env["PLATFORM"] != "win32" and "WESNOTH_PREFIX='\"$prefix\"'" or None))
 
 libwesnoth_extras = client_env.Library("wesnoth_extras", wesnoth_sources)