llvmPackages.libcxx: unset _LIBCPP_USE_AVAILABILITY_APPLE

[veprbl]

Should fix a common regression from #126411
Can go to staging or staging-next depending on how fast this can be reviewed, or whether we have time for a quick darwin stdenv rebuild.
cc #148396

[vcunat]

(I generally know almost nothing about Apple-specific stuff.)

[veprbl]

(I generally know almost nothing about Apple-specific stuff.)

I don't think many of us feel like they do :) In this instance I was pinging you because: you are watching over the staging-next regressions. Also I need help deciding where this change should go this or next staging iteration, and you probably have a better idea about timeline and amount of acceptable breakages for this.

[vcunat]

I'm not sure how bad these issues are. Hydra isn't very busy now, but the current staging-next iteration has been running for relatively long time (over two weeks) and full x86_64-darwin rebuild would surely add at least several days.

[risicle]

(am attempting to build the packages which have had their tweaks removed...)

[veprbl]

Result of nixpkgs-review pr 151151 run on x86_64-darwin 1

1 package built:

• nix

[risicle]

Not entirely sure how it's related but I get:

builder for '/nix/store/sfjsywkwvyfhc4dbvmm3fwllajl1l7c3-automake-1.16.5.drv' failed with exit code 2; last 10 log lines:
    GEN      doc/aclocal.1
    GEN      lib/Automake/Config.pm
    GEN      runtest
    GEN      t/ax/cc-no-c-o
    GEN      t/ax/shell-no-trail-bslash
    GEN      t/ax/test-defs.sh
    GEN      doc/automake-1.16.1
  help2man: can't get `--help' info from automake-1.16
  Try `--no-discard-stderr' if option outputs to stderr
  make: *** [Makefile:3743: doc/automake-1.16.1] Error 127

on macos 10.15. This is from a nixpkgs-review started approx 24h ago when staging-next head was 2add175

[veprbl]

@risicle I think saw this error on x86_64-linux recently. Although I don't remember what that was about, could be that I just worked it around by enabling sandboxing.

[veprbl]

Let's pull the trigger on this. The issue affects compilation with never standards, so users will definitely be affected if we don't fix it now.