GNU Makefile变量赋值=，？=，：=和+ =有什么区别？

问题：

Can anybody give a clear explanation of how variable assignment really works in Makefiles. 任何人都可以清楚地解释变量赋值在Makefiles中是如何工作的。

What is the difference between : 之间有什么区别：

 VARIABLE = value
 VARIABLE ?= value
 VARIABLE := value
 VARIABLE += value

I have read the section in GNU Make's manual, but it still doesn't make sense to me. 我已经阅读了GNU Make手册中的部分 ，但是对我来说仍然没有意义。

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解决方案：

参考一： https://stackoom.com/question/1smU/GNU-Makefile变量赋值-和-有什么区别
参考二： https://oldbug.net/q/1smU/What-is-the-difference-between-the-GNU-Makefile-variable-assignments-and

来源：oschina

链接：https://my.oschina.net/u/3797416/blog/4332177