pgsql: Remove configure probe for fdatasync.

Remove configure probe for fdatasync.

fdatasync() is in SUSv2, and all targeted Unix systems have it.  We have
a replacement function for Windows.

We retain the probe for the function declaration, which allows us to
supply the mysteriously missing declaration for macOS, and also for
Windows.  No need to keep a HAVE_FDATASYNC macro around.

Also rename src/port/fdatasync.c to win32fdatasync.c since it's only for
Windows.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CA+hUKGJ3LHeP9w5Fgzdr4G8AnEtJ=z=p6hGDEm4qYGEUX5B6fQ@mail.gmail.com
Discussion: https://postgr.es/m/CA%2BhUKGJZJVO%3DiX%2Beb-PXi2_XS9ZRqnn_4URh0NUQOwt6-_51xQ%40mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/d2e150831af85fd30742f551a497db6639d91d0b

Modified Files
--------------
configure                                  | 14 +++++++-------
configure.ac                               |  3 +--
src/backend/access/transam/xlog.c          |  4 ----
src/backend/storage/file/fd.c              |  8 --------
src/bin/pg_test_fsync/pg_test_fsync.c      |  4 ----
src/include/access/xlogdefs.h              |  7 ++-----
src/include/pg_config.h.in                 |  3 ---
src/include/port/freebsd.h                 |  2 --
src/include/port/win32_port.h              |  8 --------
src/port/{fdatasync.c => win32fdatasync.c} |  6 +++---
src/tools/msvc/Mkvcbuild.pm                |  3 ++-
src/tools/msvc/Solution.pm                 |  1 -
12 files changed, 15 insertions(+), 48 deletions(-)