Alzrius
The EN World kitten
But if a DM has a homebrew where there's a god of sun and magic, he has a means of building that faith for the cleric.
That's only going to happen, though, if there's a stroke of coincidence, or if the GM is (at least partially) building the world to suit the desires of his players. While that can be an aspect of world-building, it's not something a lot of GMs I know do. Even if they did, that pre-packaging of a player's concepts into the game world tends to be good only as long as the initial PCs are still around; when they die or are retired, new characters are brought in, and the world can't be so easily built to suit them.
For example, I was once running a campaign, and the guy playing a barbarian started making up things about his native tribe; their region, their history, their religion, etc., and I really felt like he was infringing on my milieu.
As the GM, I have a motto: nobody touches my milieu.