[m-rev.] for review: Use value of MACOSX_DEPLOYMENT_TARGET by default.

[Julien Fischer]

On Tue, 29 Nov 2022, Peter Wang wrote:

> On MacOS, make configure use the value of MACOSX_DEPLOYMENT_TARGET as
> the default deployment target (if set), otherwise falling back to the
> host system version.
>
> configure.ac:
>    As above.
>
> README.MacOS:
> NEWS:
>    Document the change.

That's fine.

Julien.