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3.4 The Variable `MAKEFILES'
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If the environment variable `MAKEFILES' is defined, `make' considers
its value as a list of names (separated by whitespace) of additional
makefiles to be read before the others. This works much like the
`include' directive: various directories are searched for those files
(*note Including Other Makefiles: Include.). In addition, the default
goal is never taken from one of these makefiles (or any makefile
included by them) and it is not an error if the files listed in
`MAKEFILES' are not found.
The main use of `MAKEFILES' is in communication between recursive
invocations of `make' (*note Recursive Use of `make': Recursion.). It
usually is not desirable to set the environment variable before a
top-level invocation of `make', because it is usually better not to
mess with a makefile from outside. However, if you are running `make'
without a specific makefile, a makefile in `MAKEFILES' can do useful
things to help the built-in implicit rules work better, such as
defining search paths (*note Directory Search::).
Some users are tempted to set `MAKEFILES' in the environment
automatically on login, and program makefiles to expect this to be done.
This is a very bad idea, because such makefiles will fail to work if
run by anyone else. It is much better to write explicit `include'
directives in the makefiles. *Note Including Other Makefiles: Include.
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3.4 变量 MAKEFILES
如果 环境变量 MAKEFILES 被定义了，那么 make 会在读取其他 makefile之前，先读取 MAKEFILES定义的列表中的名字(由空格分隔)。这有点像 include 指令：各目录会被搜索(*note Including Other Makefiles:Include.)。并且，缺省的终点也不会从这些makefile(或被其包含的makefile)中获得,如果找不到相应的 makefile， 也不会报错。
'MAKEFILES' 的主要用途是在 递归调用(*note Recursive Use of 'make') make 时进行通信。
通常在顶层调用 make 之前，是不推荐设置 此环境变量的，因为通常最好不要和外面的makefile混淆。
但是如果你运行 make 时，没有指定一个特定的 makefile, 那么 MAKEFILES 中定义的 makefile可以做一些有益的事情来帮助使隐式规则运行得更好，例如定义搜索路径(*note Directory Search)
有些用户试图在设置在 登录后自动设置 MAKEFILES 环境变量，并且编排 makefile使得此种设定生效。
这是一个坏主意，因为如此一来，它同时被其他人运行时，就会出错退出。更好的办法是在 makefile中写include 指令。 *Note Including Other Makefiles: Include.
后文待续
本文转自健哥的数据花园博客园博客，原文链接：http://www.cnblogs.com/gaojian/archive/2012/09/14/2684802.html，如需转载请自行联系原作者
