Jesse, as others have mentioned, I recommend bison/flex. I'm not sure the license that comes with code generated *by* bison/flex (I've only used it in companies that use it for internal tools, so I don't believe we had to worry about licensing restrictions... IANAL tho), so you should probably check that out.
That said, roxtar asks a very intelligent question. What are you attempting to do? There might be an easier way of going about it if we know your situation or goal, as well as your constraints.
That said, there's a site here that has the grammar for C in Lexx/Yacc http://www.lysator.liu.se/c/c-faq/c-17.html#17-25. At least that should cut some work out for you.