hurd-i386 does not define IOV_MAX

--- a/src/include/port/pg_iovec.h
+++ b/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 +29,11 @@ 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. */
 #define PG_IOV_MAX Min(IOV_MAX, 32)