Fix configure check for %z printf support after INT64_MODIFIER changes.

The PGAC_FUNC_SNPRINTF_SIZE_T_SUPPORT test was broken by
ce486056. Among others it made the UINT64_FORMAT macro to be
defined in c.h, instead of directly being defined by configure.

This lead to the replacement printf being used on all platforms for a
while. Which seems to work, because this was only used due to
different profiles ;)

Fix by relying on INT64_MODIFIER instead.

config/c-library.m4

@@ -327,7 +327,8 @@ int main()
    */
   bufz[0] = '\0';  /* in case snprintf fails to emit anything */
   snprintf(bufz, sizeof(bufz), "%zu", ~((size_t) 0));
-  snprintf(buf64, sizeof(buf64), UINT64_FORMAT, (PG_INT64_TYPE) ~((size_t) 0));
+  snprintf(buf64, sizeof(buf64), "%" INT64_MODIFIER "u",
+    (unsigned PG_INT64_TYPE) ~((size_t) 0));
   if (strcmp(bufz, buf64) != 0)
     return 1;
   return 0;

configure

@@ -13201,7 +13201,8 @@ int main()
    */
   bufz[0] = '\0';  /* in case snprintf fails to emit anything */
   snprintf(bufz, sizeof(bufz), "%zu", ~((size_t) 0));
-  snprintf(buf64, sizeof(buf64), UINT64_FORMAT, (PG_INT64_TYPE) ~((size_t) 0));
+  snprintf(buf64, sizeof(buf64), "%" INT64_MODIFIER "u",
+    (unsigned PG_INT64_TYPE) ~((size_t) 0));
   if (strcmp(bufz, buf64) != 0)
     return 1;
   return 0;