introduce ZSTD_USE_C90_QSORT

[Cyan4973]

in order to compile successfully when qsort_r() isn't supported and this is not detectable at compile time,
such as, for example, older versions of musl.

Fixes #4311

[iucoen]

Line 294, 310 should be

#elif defined(_GNU_SOURCE) && defined(ZSTD_USE_C90_QSORT)

Otherwise I don't think this code compiles.

[Cyan4973]

I see your point (though it's missing a !).

Interestingly, there is a musl compilation test in CI that employs this build macro:
https://github.com/facebook/zstd/blob/dev/.github/workflows/dev-short-tests.yml#L696
and it has been running fine so far.
Specifically, it detected the prototype mismatch, but just generated a warning, so the compilation test still completed successfully. And running the binary also works fine.

Anyway, seems like the test needs to be reinforced.

[iucoen]

Anyway, seems like the test needs to be reinforced.

It's a warning:

/root/zstd/lib//dictBuilder/cover.c:341:45: warning: passing argument 4 of 'qsort' from incompatible pointer type [-Wincompatible-pointer-types]
  341 |           (ctx->d <= 8 ? &COVER_strict_cmp8 : &COVER_strict_cmp));
      |           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~
      |                                             |
      |                                             int (*)(const void *, const void *, void *)
I

Run with -Werror?

[Cyan4973]

Sure, but also the current runtime test would miss the consequences of this signature mismatch (still completes successfully), so it needs an update too.