flex_counter() is used by overflow.h and needed for "flex allocations".
It is either a void * typed 0 (NULL) (like this for _Generic checks),
or uses builtin_counted_by_ref.
The latter was added to gcc and llvm fairly recently and while for gcc
the __has_builtin() check suffices, clang had parts broken until recently
so needs an extra check for the next major version. The fixed hash is
currently not part of any tag to use, so we play it save (and hope 23
will have it). It will be a while until we will see the builting to be
used but at least we will be prepared for it. See inline comments for
the commit hashes and versions which added the feature.
Sponsored by: The FreeBSD Foundation
MFC after: 3 days