这是因为实际的源代码和变量绑定可以更改，这是建立在环境或者构建宏预处理程序指令的时间定义的基础上的。
This is because the actual source code and the variable binding can change, based on the environment or the build time definitions of macros and preprocessor commands.
它在系统设计上的重要技术成果之一是实现了高效率的宏指令处理方案。
One of the significant technical achievements in system design lays on the implementation of efficient macroinstruction processing scheme.
使用指令可以有条件地处理生成文件，显示错误消息，包括其他生成文件，取消定义宏以及打开或关闭某些选项。
Using directives, you can conditionally process your makefile, display error messages, include other makefiles, undefine a macro, and turn certain options on or off.
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