Make sure IOV_MAX is defined.

We stopped defining IOV_MAX on non-Windows systems since 75357ab under
the assumption that every non-Windows system defines it in limits.h
already. However, the GNU (Hurd) system does not define this limit.

As POSIX does not mandate IOV_MAX be defined, define it to 16 if it is
undefined.

Co-authored-by: Christoph Berg

Author: mbanck

src/include/port/pg_iovec.h

@@ -21,9 +21,6 @@
 
 #else
 
-/* POSIX requires at least 16 as a maximum iovcnt. */
-#define IOV_MAX 16
-
 /* Define our own POSIX-compatible iovec struct. */
 struct iovec
 {
@@ -33,6 +30,11 @@ struct iovec
 
 #endif
 
+/* POSIX requires at least 16 as a maximum iovcnt. */
+#ifndef IOV_MAX
+#define IOV_MAX 16
+#endif
+
 /*
  * Define a reasonable maximum that is safe to use on the stack in arrays of
  * struct iovec and other small types.  The operating system could limit us to