Well, I'd say that that Capitalism is the practice of the owners of capital and providers of labor (which might be overlapping sets) getting together to sell items for a profit, with said profits apportioned between labor and capital somehow. Which is clearly not how open source software works.
Capitalism is entirely compatible with free markets and since it seems to be the best way to produce rivalrous, excludable goods I'd expect to see it in any free market unless humans someday invent something better. On the other hand you can easily imagine a non-free market capitalist system, say where the state grants a monopoly on every good to some company or other, or maybe just auctions the monopolies off. I'm not sure how you can call capitalism a political system, since it's existed under a wide variety of political systems ever since it's time coexisting with feudalism.