Context-free grammar for C

You could always use Annex A of the C11 standard itself. The freely available draft standard will work for your purposes, at http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf .

This is an ANSI-C grammar, updated to C11 standard. Should serve your purpose.

http://www.quut.com/c/ANSI-C-grammar-y.html