Make sure IOV_MAX is defined.

We stopped defining IOV_MAX on non-Windows systems in 75357ab94, on
the assumption that every non-Windows system defines it in <limits.h>
as required by X/Open.  GNU Hurd, however, doesn't follow that
standard either.  Put back the old logic to assume 16 if it's
not defined.

Author: Michael Banck <mbanck@gmx.net>
Co-authored-by: Christoph Berg <myon@debian.org>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/6862e8d1.050a0220.194b8d.76fa@mx.google.com
Discussion: https://postgr.es/m/6846e0c3.df0a0220.39ef9b.c60e@mx.google.com
Backpatch-through: 16

src/include/port/pg_iovec.h

@@ -20,9 +20,6 @@
 
 #else
 
-/* POSIX requires at least 16 as a maximum iovcnt. */
-#define IOV_MAX 16
-
 /* Define our own POSIX-compatible iovec struct. */
 struct iovec
 {
@@ -32,6 +29,15 @@ struct iovec
 
 #endif
 
+/*
+ * If <limits.h> didn't define IOV_MAX, define our own.  X/Open requires at
+ * least 16.  (GNU Hurd apparently feel that they're not bound by X/Open,
+ * because they don't define this symbol at all.)
+ */
+#ifndef IOV_MAX
+#define IOV_MAX 16
+#endif
+
 /* Define a reasonable maximum that is safe to use on the stack. */
 #define PG_IOV_MAX Min(IOV_MAX, 32)