Annoying build warnings from latest Apple toolchain

Since updating to Xcode 15.0, my macOS machines have been
spitting a bunch of linker-generated warnings.  There
seems to be one instance of

ld: warning: -multiply_defined is obsolete

for each loadable module we link, and some program links complain

ld: warning: ignoring duplicate libraries: '-lpgcommon', '-lpgport'

You can see these in the build logs for both sifaka [1] and
indri [2], so MacPorts isn't affecting it.

I'd held out some hope that this was a transient problem due to
the underlying OS still being Ventura, but I just updated another
machine to shiny new Sonoma (14.0), and it's still doing it.
Guess we gotta do something about it.

We used to need "-multiply_defined suppress" to suppress other
linker warnings.  I tried removing that from the Makefile.shlib
recipes for Darwin, and those complaints go away while no new
ones appear, so that's good --- but I wonder whether slightly
older toolchain versions will still want it.

As for the duplicate libraries, yup guilty as charged, but
I think we were doing that intentionally to satisfy some other
old toolchains.  I wonder whether removing the duplication
will create new problems.

            regards, tom lane

[1] https://buildfarm.postgresql.org/cgi-bin/show_stage_log.pl?nm=sifaka&dt=2023-09-26%2021%3A09%3A01&stg=build
[2] https://buildfarm.postgresql.org/cgi-bin/show_stage_log.pl?nm=indri&dt=2023-09-26%2021%3A42%3A17&stg=build