[core] fix getenv namespace error with libc++

[ferdymercury]

Fixes #19850

[guitargeek]

Thanks a lot! Will you take care of the backport to 6.36, or should I do it?

[ferdymercury]

Thanks a lot! Will you take care of the backport to 6.36, or should I do it?

I could do it. The question to me before is:

• There are more ::getenv throughout the codebase without std:: prepended. Not sure if they should also be changed, for consistency or if that will break things on Windows or who knows what.
• The user also reports a failure with cxxmodules after fixing this, (see same issue)

[guitargeek]

Good points!

• we should use std::getenv everywhere. The idea of moden C++ is to not use the C headers like stdlib.h, but instead use the standard library headers like cstdlib. See also this clang tidy check:
  https://clang.llvm.org/extra/clang-tidy/checks/modernize/deprecated-headers.html
  So it's probably a good opportunity to use std::getenv all the way 👍. Can we do it in this PR?
  This should also work on Windows, as the functions do the same thing.
• I have seen that, that's a shame... Let's continue the discussion in that PR and ping our macOS expert @couet when we're stuck

See also:

• https://en.cppreference.com/w/cpp/header/cstdlib.html
• https://en.cppreference.com/w/c/header/stdlib.html

[github-actions]

Test Results

    20 files      20 suites   3d 22h 54m 48s ⏱️
 3 663 tests  3 661 ✅   0 💤 2 ❌
71 547 runs  71 439 ✅ 106 💤 2 ❌

For more details on these failures, see this check.

Results for commit 0c1f275.

♻️ This comment has been updated with latest results.

[ferdymercury]

Can we do it in this PR?

Done as suggested

[guitargeek]

Nice, thank you very much!