uses_from_macos: allow libxcrypt

[Bo98]

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Homebrew/homebrew-core#99367

[BrewTestBot]

Review period ended.

[Bo98]

🐔 🥚

[MikeMcQuaid]

Fine with me but is this technically provided by macOS or should it be ALLOWED_USES_FROM_MACOS_DEPS (or something else)?

[Bo98]

So this specific implementation isn't provided by macOS, nor does the dylib or header shadow anything from macOS by filename, but the functions that libxcrypt offer (e.g. crypt) are provided by macOS in libSystem.

I think making it keg-only is safer since there are some configure scripts that will do AC_CHECK_HEADERS(crypt.h) and either assume glibc or prioritise using that header over anything in the system.