While it's not exactly about forest animals making houses from war ruins, the cartoon does feature:
* Forest animals: The characters include a rabbit, a squirrel, a deer, and a bear, who all live in the forest.
* War ruins: The forest setting is explicitly stated to be near a war-torn area, with the animals' homes being makeshift creations from the debris of war.
* Christmas celebration: The story is set around Christmas, with the animals decorating their homes and celebrating the holiday together.
Though not a direct match for your description, this classic cartoon shares many thematic similarities.
It's important to note that the term "Merrie Melodies" is a broad label for many Warner Bros. animated shorts. There may be other cartoons within this collection that more closely match your description, but this one is the most fitting based on your details.