improve platform

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diff --git a/xmake/core/base/singleton.lua b/xmake/core/base/singleton.lua
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+++ b/xmake/core/base/singleton.lua
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+--!A cross-platform build utility based on Lua
+--
+-- Licensed to the Apache Software Foundation (ASF) under one
+-- or more contributor license agreements.  See the NOTICE file
+-- distributed with this work for additional information
+-- regarding copyright ownership.  The ASF licenses this file
+-- to you under the Apache License, Version 2.0 (the
+-- "License"); you may not use this file except in compliance
+-- with the License.  You may obtain a copy of the License at
+--
+--     http://www.apache.org/licenses/LICENSE-2.0
+--
+-- Unless required by applicable law or agreed to in writing, software
+-- distributed under the License is distributed on an "AS IS" BASIS,
+-- WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+-- See the License for the specific language governing permissions and
+-- limitations under the License.
+-- 
+-- Copyright (C) 2015 - 2018, TBOOX Open Source Group.
+--
+-- @author      ruki
+-- @file        singleton.lua
+--
+
+-- define module
+local singleton = singleton or {}
+
+-- get a singleton instance and create it if not exists
+--
+-- e.g. 
+--
+-- local instance = singleton.get("key")
+-- 
+-- function get_xxx()
+--
+--     -- get instance
+--     local instance, inited = singleton.get(get_xxx)
+--     if inited then
+--          return instance
+--     end
+--
+--     -- init instance
+--     instance.xxx = 1
+--     return instance
+-- end
+--
+function singleton.get(key)
+
+    -- get key
+    key = tostring(key)
+
+    -- get all instances 
+    local instances = singleton.instances()
+
+    -- get singleton instance
+    local inited = true
+    local instance = instances[key]
+    if instance == nil then
+
+        -- mark as not inited
+        inited = false
+
+        -- init instance
+        instance = {}
+
+        -- save this instance
+        instances[key] = instance
+    end
+    return instance, inited
+end
+
+-- get all singleton instances
+function singleton.instances()
+    local instances = singleton._INSTANCES
+    if not instances then
+        instances = {}
+        singleton._INSTANCES = instances
+    end
+    return instances
+end
+
+-- return module: singleton
+return singleton