move SDL_SavePNG to the SDL compilation group, on cmake also

This ports over this commit for scons:
ff857d1

src/CMakeLists.txt

@@ -394,7 +394,7 @@ if(LIBDBUS_FOUND)
 	)
 endif()
 
-if(PNG_FOUND)
+if(ENABLE_LIBPNG AND PNG_FOUND)
 	set_source_files_properties(
 		SDL_SavePNG/savepng.cpp
 		PROPERTIES
@@ -415,6 +415,15 @@ set(wesnoth-sdl_SRC
 	xBRZ/xbrz.cpp
 )
 
+# If requested, compile and link this file also to be able to save png files
+if(ENABLE_LIBPNG AND PNG_FOUND)
+	set(wesnoth-sdl_SRC
+		${wesnoth-sdl_SRC}
+		SDL_SavePNG/savepng.cpp
+	)
+endif(ENABLE_LIBPNG AND PNG_FOUND)
+
+
 add_library(wesnoth-sdl
 	${LIBRARY_TYPE}
 	EXCLUDE_FROM_ALL
@@ -1065,14 +1074,6 @@ if(LIBDBUS_FOUND)
 	)
 endif(LIBDBUS_FOUND)
 
-# If requested, compile and link this file also to be able to save png files
-if(ENABLE_LIBPNG AND PNG_FOUND)
-	set(libwesnoth-game_STAT_SRC
-		${libwesnoth-game_STAT_SRC}
-		SDL_SavePNG/savepng.cpp
-	)
-endif(ENABLE_LIBPNG AND PNG_FOUND)
-
 if(ENABLE_GAME AND ENABLE_TESTS)
 	set(libwesnoth-game_STAT_SRC
 		${libwesnoth-game_STAT_SRC}