introduce static mut IS_POSIXLY_CORRECT

[xtqqczze]

No description provided.

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/tail/retry. tests/tail/retry is passing on 'main'. Maybe you have to rebase?
Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/tail/symlink (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/tail/tail-n0f is now being skipped but was previously passing.

[RenjiSann]

Why would you introduce so much unsafe here ? Is it motivated by a significant gain in performance ?

[codspeed-hq]

Merging this PR will degrade performance by 23.82%

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

❌ 1 regressed benchmark
✅ 288 untouched benchmarks
⏩ 66 skipped benchmarks¹

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Memory	cp_recursive_deep_tree[(120, 4)]	532.7 KB	699.2 KB	-23.82%

---

_{Comparing xtqqczze:static-posixly-correct3 (6dc2b99) with main (896c685)}

Footnotes

1. 66 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩