When testing some configurations of compilers and OSes, I noticed
that clang(1) 13 on Debian caused a function to be compiled but
unused, and the compiler triggered a compile error.

To avoid that error, use __attribute__((__unused__)).  Let's call
our wrapper NXT_MAYBE_UNUSED, since it describes itself more
precisely than the GCC attribute name.  It's also the name that
C2x (likely C23) has given to the standard attribute, which is
[[maybe_unused]], so it's also likely to be more readable because
of that name being in ISO C.

Thanks to 洪志道 (Hong Zhi Dao).