GNU make manual 翻译( 一百五十八)

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   The previous example shows two levels of nesting, but any number of
levels is possible.  For example, here are three levels:

     x = y
     y = z
     z = u
     a := $($($(x)))

Here the innermost `$(x)' expands to `y', so `$($(x))' expands to
`$(y)' which in turn expands to `z'; now we have `$(z)', which becomes
`u'.

   References to recursively-expanded variables within a variable name
are reexpanded in the usual fashion.  For example:

     x = $(y)
     y = z
     z = Hello
     a := $($(x))

defines `a' as `Hello': `$($(x))' becomes `$($(y))' which becomes
`$(z)' which becomes `Hello'.

   Nested variable references can also contain modified references and
function invocations (*note Functions for Transforming Text:
Functions.), just like any other reference.  For example, using the
`subst' function (*note Functions for String Substitution and Analysis:
Text Functions.):

     x = variable1
     variable2 := Hello
     y = $(subst 1,2,$(x))
     z = y
     a := $($($(z)))

eventually defines `a' as `Hello'.  It is doubtful that anyone would
ever want to write a nested reference as convoluted as this one, but it
works: `$($($(z)))' expands to `$($(y))' which becomes `$($(subst
1,2,$(x)))'.  This gets the value `variable1' from `x' and changes it
by substitution to `variable2', so that the entire string becomes
`$(variable2)', a simple variable reference whose value is `Hello'.

前面的例子展示了两层的嵌套，但是任何层的嵌套都是可能的。例如，这里有三个层次：

x = y
y = z
z = u
a := $($($(x)))

这里，最里层的 `$(x)' 扩展到  `y'，于是 `$($(x))' 扩展到`$(y)'，进而扩展到`z'; 现在我们拥有`$(z)'，它将变为 `u'。

在一个变量里面，对递归式扩展的变量的参照会合正常的扩展一样。例如:

x = $(y)
y = z
z = Hello
a := $($(x))

定义 `a' 为 `Hello': `$($(x))' 为 `$($(y))' ，然后转化为，`$(z)' ，再转化为 `Hello'。

嵌套的变量参照也能包含变化的参照和函数激活(*note Functiosn for Transforming Text: Functions)，就像其他的参照一样。例如，使用 subst函数(*Functions for String Substitution and Analysis: Text Functions.):

x = variable1
variable2 := Hello
y = $(subst 1,2,$(x))
z = y
a := $($($(z)))

最终定义 `a' 为 `Hello'。是否会有人写出这样的嵌套式的参照是令人怀疑的，但是它确实可以工作：

`$($($(z)))' 扩展为 `$($(y))' ，它变成 `$($(subst
1,2,$(x)))'. 然后从x 来获得 `variable1' ,再通过变换，把它变成 `variable2', 这样整个字符串就变成了
`$(variable2)', 成为一个简单的变量参照，其值为 `Hello'。

后文待续