Re: pg_preadv() and pg_pwritev()

I wrote:
> Sergey Shinderuk <s(dot)shinderuk(at)postgrespro(dot)ru> writes:
>>>> note: 'pwritev' has been marked as being introduced in macOS 11.0 here,
>>>> but the deployment target is macOS 10.15.0

> The symptoms sound consistent with using bleeding-edge Xcode on a
> Catalina machine ... please report exact OS and Xcode versions.

I can reproduce these warnings on Big Sur + Xcode 12.3 by doing

export MACOSX_DEPLOYMENT_TARGET=10.15

before building; however the executable runs anyway, which I guess
is unsurprising. AFAICS from config.log, configure has no idea
that anything is wrong.

(BTW, at least the rather-old version of ccache I'm using does not
seem to realize that that environment variable is significant;
I had to clear ~/.ccache to get consistent results.)

We've had issues before with weird results from Xcode versions
newer than the underlying OS. In the past we've been able to
work around that, but I'm not sure that I see a way here.

regards, tom lane