为了解决project依赖

makefile的一般格式如下:

target: prerequisites
    recipe

target, 文件或命令,file or an action(.PHONY)
prerequisites, target依赖的文件
recipe,得到文件需要的命令,recipes say how to update the target file.

When a target is a file, it needs to be recompiled or relinked if any of its prerequisites change.