add note about rdma-credentials being suggested only on Cray XC systems

configure.ac

@@ -540,7 +540,7 @@ AS_IF([test "x$enable_cray_drc" = "xyes"],
      AC_MSG_RESULT([DRC support found])
     else
      AC_MSG_NOTICE([no libdrc support])
-     AC_MSG_WARN([You seem to be running on a Cray system; consider loading module rdma-credentials])
+     AC_MSG_WARN([You seem to be running on a Cray system; consider loading module rdma-credentials if running on an XC 30/40/50 system])
     fi
    ],
    [AC_MSG_RESULT([Not a Cray system, or no Cray PE installed])])],