pgsql: Probe for preadv/pwritev in a more macOS-friendly way.

Probe for preadv/pwritev in a more macOS-friendly way.

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/f014b1b9bb8eda4e82c1805969dbae2b07b7d54d

Modified Files
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configure                   | 166 +++++++++++++++++++++++++++++++++++++-------
configure.ac                |  11 ++-
src/include/pg_config.h.in  |  14 ++--
src/include/port/pg_iovec.h |   4 +-
src/tools/msvc/Solution.pm  |   4 +-
5 files changed, 161 insertions(+), 38 deletions(-)