That's not really a problem though. Pretty much any non-trivial real-world codebase isn't pure standard C, many are absolutely riddled with non-standard extensions and it works just fine (you'll need to build and test on all supported compilers and platforms anyway).

For that Clang extension above it looks like it's possible to annotate source code without breaking compilers that don't support the extension by defining a handful of dummy macros.

IMHO the actual strength of C is that compilers can (and do) explore beyond the standard on their own.

Despite urban myths plenty of languages have implementation specific extensions, it isn't some feature exclusive to C dialects.

It is definitely a problem when said extensions don't have a counterpart in other compilers, specially a pervasive feature like bounds checking.