Apple's mechanism for dealing with functions that are available
in only some OS versions confuses AC_CHECK_FUNCS, and therefore
AC_REPLACE_FUNCS. We can use AC_CHECK_DECLS instead, so long as
we enable -Werror=unguarded-availability-new. This allows people
compiling for macOS to control whether or not preadv/pwritev are
used by setting MACOSX_DEPLOYMENT_TARGET, rather than supplying
a back-rev SDK. (Of course, the latter still works, too.)
James Hilliard
Discussion: https://postgr.es/m/20210122193230.25295-1-james.hilliard1@gmail.com
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