Move pg_upgrade's Windows link() implementation to AC_REPLACE_FUNCS

This way we can make use of it in other components as well, and it
fits better with the rest of the build system.

Discussion: https://www.postgresql.org/message-id/flat/72fff73f-dc9c-4ef4-83e8-d2e60c98df48%402ndquadrant.com

src/port/link.c

@@ -0,0 +1,35 @@
+/*-------------------------------------------------------------------------
+ *
+ * link.c
+ *
+ * Portions Copyright (c) 1996-2020, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ *
+ * IDENTIFICATION
+ *	  src/port/link.c
+ *
+ *-------------------------------------------------------------------------
+ */
+
+#include "c.h"
+
+#ifdef WIN32
+
+int
+link(const char *src, const char *dst)
+{
+	/*
+	 * CreateHardLinkA returns zero for failure
+	 * https://docs.microsoft.com/en-us/windows/win32/api/winbase/nf-winbase-createhardlinka
+	 */
+	if (CreateHardLinkA(dst, src, NULL) == 0)
+	{
+		_dosmaperr(GetLastError());
+		return -1;
+	}
+	else
+		return 0;
+}
+
+#endif