Since God is the creator of man any valid life philosophy should include the mention of a supreme being. A life philosophy that tries to explain meaning would be incomplete without there being a mention of God in some context, since a majority of humanity believes in a higher power, leaving out God would make the content incomplete.
No, It should not mention god in the meaning of life philosophy, since god is nothing but an imaginary figure that people use to explain things that they can not explain. They need to throw some science in there and some good science theories that explain how we got here.
Although many individuals find meaning in their faith, an agnostic or atheist can also lead a meaningful life. Therefore, a valid meaning of life philosophy does not have to mention God. For example, a scientist may find meaning in discovering a cure for illness, and an artist may find meaning in creating beauty. Their life is still meaningful, regardless of their beliefs.
God is irrelevant. Philosophy seeks to explain and the nature of being as it is. The concept of "God" is outside of this, and it is alien to atheists who lead good, philosophically just lives. Saying "God needs to be in philosophy" is just another case of religious zealots trying exert control over something they have no right to.
I don't think you need to mention god in philosophy to make it a valid meaning because not every person believes in god and not everything should come back to god when talking about philosophy. You need to think of the people who have a religion other then a god believing religion.