Use C11 in the default's gcc standard.
Use C11 in the default's gcc standard.

The `stdalign.h` header is from the relatively recent C11 standard. To
get the tests to pass on non-bleeding-edge gcc versions, force `gnu11`
to be the standard in the `Default` impl for `Probe`.

quantheory/probe-c-api,quantheory/probe-c-api