继续翻译
`.EXPORT_ALL_VARIABLES'
Simply by being mentioned as a target, this tells `make' to export
all variables to child processes by default. *Note Communicating
Variables to a Sub-`make': Variables/Recursion.
通过简单地声明 .EXPORT_ALL_VARIABLES ，可以告诉 make 来到出所有的变量给 子进程。
*Note Communicating Variables to a Sub-make: Variables/Recursion.
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