Remove configure probe for sys/uio.h.

<sys/uio.h> is in SUSv2, and all targeted Unix system have it, so we
might as well drop the probe (in fact we never really needed this one).
It's where struct iovec is defined, and as a common extension, it's also
where non-standard preadv() and pwritev() are declared on systems that
have them.

We should also be able to assume that IOV_MAX is defined on Unix.

To spell out what our pg_iovec.h header does for the OSes in the build
farm as of today:

  Windows: our own struct and functions
  Solaris, Cygwin: <sys/uio.h>'s struct, our own functions
  Every other Unix: <sys/uio.h>'s struct and functions

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/CA%2BhUKG%2BL_3brvh%3D8e0BW_VfX9h7MtwgN%3DnFHP5o7X2oZucY9dg%40mail.gmail.com

configure

@@ -13874,7 +13874,7 @@ $as_echo "#define HAVE_STDBOOL_H 1" >>confdefs.h
 fi
 
 
-for ac_header in atomic.h copyfile.h execinfo.h getopt.h ifaddrs.h langinfo.h mbarrier.h net/if.h netinet/tcp.h sys/epoll.h sys/event.h sys/ipc.h sys/personality.h sys/prctl.h sys/procctl.h sys/resource.h sys/select.h sys/sem.h sys/shm.h sys/signalfd.h sys/sockio.h sys/ucred.h sys/uio.h sys/un.h termios.h ucred.h
+for ac_header in atomic.h copyfile.h execinfo.h getopt.h ifaddrs.h langinfo.h mbarrier.h net/if.h netinet/tcp.h sys/epoll.h sys/event.h sys/ipc.h sys/personality.h sys/prctl.h sys/procctl.h sys/resource.h sys/select.h sys/sem.h sys/shm.h sys/signalfd.h sys/sockio.h sys/ucred.h sys/un.h termios.h ucred.h
 do :
   as_ac_Header=`$as_echo "ac_cv_header_$ac_header" | $as_tr_sh`
 ac_fn_c_check_header_mongrel "$LINENO" "$ac_header" "$as_ac_Header" "$ac_includes_default"

configure.ac

@@ -1467,7 +1467,6 @@ AC_CHECK_HEADERS(m4_normalize([
 	sys/signalfd.h
 	sys/sockio.h
 	sys/ucred.h
-	sys/uio.h
 	sys/un.h
 	termios.h
 	ucred.h

src/include/pg_config.h.in

@@ -538,9 +538,6 @@
 /* Define to 1 if you have the <sys/ucred.h> header file. */
 #undef HAVE_SYS_UCRED_H
 
-/* Define to 1 if you have the <sys/uio.h> header file. */
-#undef HAVE_SYS_UIO_H
-
 /* Define to 1 if you have the <sys/un.h> header file. */
 #undef HAVE_SYS_UN_H
 

src/include/port/pg_iovec.h

@@ -13,27 +13,23 @@
 #ifndef PG_IOVEC_H
 #define PG_IOVEC_H
 
+#ifndef WIN32
+
 #include <limits.h>
-
-#ifdef HAVE_SYS_UIO_H
 #include <sys/uio.h>
-#endif
 
-/* If <sys/uio.h> is missing, define our own POSIX-compatible iovec struct. */
-#ifndef HAVE_SYS_UIO_H
+#else
+
+/* POSIX requires at least 16 as a maximum iovcnt. */
+#define IOV_MAX 16
+
+/* Define our own POSIX-compatible iovec struct. */
 struct iovec
 {
 	void	   *iov_base;
 	size_t		iov_len;
 };
-#endif
 
-/*
- * If <limits.h> didn't define IOV_MAX, define our own.  POSIX requires at
- * least 16.
- */
-#ifndef IOV_MAX
-#define IOV_MAX 16
 #endif
 
 /* Define a reasonable maximum that is safe to use on the stack. */

src/tools/msvc/Solution.pm

@@ -375,7 +375,6 @@ sub GenerateFiles
 		HAVE_SYS_STAT_H                          => 1,
 		HAVE_SYS_TYPES_H                         => 1,
 		HAVE_SYS_UCRED_H                         => undef,
-		HAVE_SYS_UIO_H                           => undef,
 		HAVE_SYS_UN_H                            => undef,
 		HAVE_TERMIOS_H                           => undef,
 		HAVE_TYPEOF                              => undef,