Consider testing __STDC_VERSION__ >= 202000L in avif.h for C compiler's [[nodiscard]] support

[wantehchang]

The [[nodiscard]] attribute will be added to C23. That's why we are testing __STDC_VERSION__ >= 202311L in avif.h.

Experiments showed that if we pass the -std=c2x or -std=gnu2x option to recent versions of GCC and Clang (including Apple Clang), the __STDC_VERSION__ macro is defined as 202000L. So we may want to change the __STDC_VERSION__ >= 202311L test to __STDC_VERSION__ >= 202000L in avif.h.

Note: If we pass the /std:clatest option to MSVC, __STDC_VERSION__ macro is defined as 202312L.

[wantehchang]

This test program prints the value of __STDC_VERSION__:

#include <stdio.h>

int main(void) {
  printf("__STDC_VERSION__ = %ldL\n", __STDC_VERSION__);
  return 0;
}

For GCC and Clang, we can use the command echo | gcc -std=c2x -dM -E - | grep __STDC_VERSION__ or echo | clang -std=c2x -dM -E - | grep __STDC_VERSION__.

[vrabaud]

According to the standard, (cf https://open-std.org/JTC1/SC22/WG14/www/docs/n3220.pdf , look for the string 201904L), 201904L should do it (if compilers implement it strictly ... but I doubt it for non-final C versions), so 202000L is fine.