mysql8: use MacPorts' libc++ on macOS ≤10.12

[chrstphrchvz]

…for std::shared_timed_mutex and std::bad_optional_access
See: https://trac.macports.org/ticket/60853

Description

Tested on

Untested.

Verification

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[macportsbot]

Notifying maintainers:
@herbygillot for port mysql8.

[chrstphrchvz]

Pinging @cjones051073 in case of interest.

[chrstphrchvz]

Now that I see 1cdb3d2, would migrating this port to libc++ work to provide std::bad_optional_access for macOS 10.12 and earlier (if there is interest in this port working on older macOS)?

[herbygillot]

Now that I see 1cdb3d2, would migrating this port to libc++ work to provide std::bad_optional_access for macOS 10.12 and earlier (if there is interest in this port working on older macOS)?

I think there is always interest in making software work with older versions of macOS, if you can figure out how.

[chrstphrchvz]

If this PR or similar approach to support macOS ≤10.12 is rejected, then https://trac.macports.org/ticket/60379 can also be closed.

[cjones051073]

In order for this approach to work at all, you need to remove the known_fail block.

[cjones051073]

tried this but it does not help]

:info:build /opt/local/var/macports/build/_Users_chris_Projects_MacPorts_ports_databases_mysql8/mysql8/work/mysql-8.0.27/storage/temptable/include/temptable/kv_store.h:76:50: error: 'shared_timed_mutex' is unavailable: introduced in macOS 10.12
:info:build       std::conditional_t<std::is_same<Lock, std::shared_timed_mutex>::value,

I think it is going to be excessively hard to support < macOS 10.13.