Re: pg_preadv() and pg_pwritev()

Hmmm ... I can further report that on Catalina + Xcode 12.0,
everything seems fine.  configure correctly detects that preadv
and pwritev aren't there:

configure:15161: checking for preadv
configure:15161: ccache gcc -o conftest -Wall -Wmissing-prototypes -Wpointer-ar\
ith -Wdeclaration-after-statement -Werror=vla -Wendif-labels -Wmissing-format-a\
ttribute -Wformat-security -fno-strict-aliasing -fwrapv -Wno-unused-command-lin\
e-argument -g -O2 -isysroot /Applications/Xcode.app/Contents/Developer/Platform\
s/MacOSX.platform/Developer/SDKs/MacOSX10.15.sdk   -isysroot /Applications/Xcod\
e.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.15.s\
dk   conftest.c -lz  -lm  >&5
Undefined symbols for architecture x86_64:
  "_preadv", referenced from:
      _main in conftest-fca7e9.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
configure:15161: $? = 1

So I'm a little confused as to why this test is failing to fail
with (I assume) newer Xcode.  Can we see the relevant part of
config.log on your machine?

            regards, tom lane