In a medium saucepan, melt the butter. Add the cumin and mustard seeds and cook over high heat until the mustard seeds begin to pop, about 30 seconds. Add the rice and diced squash and stir to coat with the butter. Add the water and salt and bring to a boil. Cover and cook over very low heat until the squash is tender and the water is completely absorbed, about 15 minutes. Remove from the heat and let the rice stand, covered, for 5 minutes. Fluff the rice with a fork and serve right away.