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/include/port.h

@@ -381,6 +381,10 @@ extern float pg_strtof(const char *nptr, char **endptr);
 #define strtof(a,b) (pg_strtof((a),(b)))
 #endif
 
+#ifndef HAVE_LINK
+extern int link(const char *src, const char *dst);
+#endif
+
 #ifndef HAVE_MKDTEMP
 extern char *mkdtemp(char *path);
 #endif