GNU make manual 翻译( 一百六十三)

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6.6 Appending More Text to Variables
====================================

Often it is useful to add more text to the value of a variable already
defined.  You do this with a line containing `+=', like this:

     objects += another.o

This takes the value of the variable `objects', and adds the text
`another.o' to it (preceded by a single space).  Thus:

     objects = main.o foo.o bar.o utils.o
     objects += another.o

sets `objects' to `main.o foo.o bar.o utils.o another.o'.

   Using `+=' is similar to:

     objects = main.o foo.o bar.o utils.o
     objects := $(objects) another.o

but differs in ways that become important when you use more complex
values.

   When the variable in question has not been defined before, `+=' acts
just like normal `=': it defines a recursively-expanded variable.
However, when there _is_ a previous definition, exactly what `+=' does
depends on what flavor of variable you defined originally.  *Note The
Two Flavors of Variables: Flavors, for an explanation of the two
flavors of variables.

6.6 给变量追加更多文本
====================================

给一个已经定义好的变量追加更多的文本经常是很有用的。你可以用一行包含+=的行来做这件事。向这样：

objects += another.o

这将获得变量objects得值，然后把another.o 追加到它后面(前面放一个空格)。因此：

objects = main.o foo.o bar.o utils.o
objects += another.o

设置 `objects' 为 `main.o foo.o bar.o utils.o another.o'。

Using `+=' 等同于:

objects = main.o foo.o bar.o utils.o

objects := $(objects) another.o

但是当你使用更加复杂的值的时候，其差异就变得重要了。

当上述问题中的变量没有被定义的时候，+=就像通常的=: 一样，它定义了一个递归的变量。但是当变量已经有了定义，+=要做的依赖于你事前定义此变量所用的风格。关于变量两种风格，参照 *Note the Tow Flavors of Variables: Flavors

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