GNU make manual 翻译(五十九)

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Conditional Directives
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Conditional directives are parsed immediately.  This means, for example, that automatic variables cannot be used in conditional directives, as automatic variables are not set until the recipe for  that rule is invoked.  If you need to use automatic variables in a  conditional directive you _must_ move the condition into the recipe and use shell conditional syntax instead.

条件性指令

条件性指令会被立即解析。这意味着，例如，自动化变量不能在 条件性指令中使用，因为自动化变量是要知道规则的片段被执行时才会被设置的。如果你需要在条件性指令中使用一个自动化变量，你必须把这个条件移动到片断中，并且用 shell 的条件语法来代替。

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