Re: Annoying build warnings from latest Apple toolchain

I wrote:
> 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 ld: warning: -multiply_defined is obsolete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 ...

I poked into this a little more. We started using "-multiply_defined
suppress" in commit 9df308697 of 2004-07-13, which was in the OS X 10.3
era. I failed to find any specific discussion of that switch in our
archives, but the commit message suggests that I probably stole it
from a patch the Fink project was carrying.

Googling finds some non-authoritative claims that "-multiply_defined"
has been a no-op since OS X 10.9 (Mavericks). I don't have anything
older than 10.15 to check, but removing it on 10.15 does not seem
to cause any problems.

So I think we can safely just remove this switch from Makefile.shlib.
The meson build process isn't invoking it either I think.

The other thing will take a bit more work ...

regards, tom lane