Starting from clang 21 (see [release notes][1]) clang also supports
`-Wunterminated-string-initialization` and the corresponding
`__attribute__((nonstring))`. Hence, also provide `NONSTRING` as the
corresponding `__attribute__((nonstring))` for clang starting with
clang version 21.
[1]: https://releases.llvm.org/21.1.0/tools/clang/docs/ReleaseNotes.html
This adds the `NONSTRING` attribute that is defined as either
`__attribute__((nonstring))` or as empty, depending on whether the
toolchain understands the `nonstring` attribute.
This allows declaring char arrays as not being a zero-terminated
c-string without cluttering the code with preprocessor conditinational
to ensure backward compatibility with compilers that do not support
`-Wunterminated-string-initialization` yet.
