It appears that [[fallthrough]] is valid in C++ but not in C. And
in some Clang versions (e.g. Clang 11 on macOS), Clang is pedantic
about this and considers it an error to use [[fallthrough]] in a
.c file such as src/util/format.c.
This changes makes gcc_fallthrough a no-op under Clang in C files.
Older clang versions don't support the GCC __attribute__ syntax. For
those, don't use anything at all, and new clang versions shall use the
standard syntax.
