adding a --keyword option to optionally change the lambda keyword to something else

in the spirit of C11 we recommend _LAmbda, of course with a capital A, you never know whether they'd some day add _Lambda to the standard already, or whether someone's already using _lambda, and we'd never want to break existing code. You know how people try to avoid generic names, like, they decide not to use the name alignof because it could become a standard, and then the standard uses _Alignof... Where's _Sizeof?