Support C11 values for -std=... option

Even without support for any new C11 features, this is useful to let
sparse run as part of the build process for a project that is just
beginning to migrate to C11.

Although there are some errors caused by lack of support for anonymous
structs and unions, it's a lot better than just saying "Unsupported C
dialect".

Signed-off-by: John Keeping <john@keeping.me.uk>