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<title>第七章 : Makefile的条件执行</title>
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<a name="chapter_07"><span style="font-size: 18.0pt; font-family: 楷体_GB2312">
第七章：</span><span lang="EN-US" style="font-size: 18.0pt">Makefile</span><span style="font-size: 18.0pt; font-family: 楷体_GB2312">的条件执行</span></a></h1>
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条件语句可以根据一个变量的值来控制</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">make</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">执行或者忽略</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">Makefile</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">的特定部分。条件语句可以是两个不同变量、或者变量和常量值的比较。要注意的是：条件语句只能用于控制</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">make</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">实际执行的</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">makefile</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">文件部分，它不能控制规则的</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">shell</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">命令执行过程。</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">Makefile</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">中使用条件控制可以做到处理的灵活性和高效性。</span></p>
<h2><a name="_gnu_make_7.1"><span style="font-size: 14pt" lang="en-us"><b>7</b></span><b><span lang="EN-US" style="font-size: 14.0pt">.1<span style="font-style: normal; font-variant: normal; font-weight: normal; font-size: 7.0pt; font-family: Times New Roman">
</span></span></b><span style="font-size: 14.0pt; font-family: 楷体_GB2312">一个例子</span></a></h2>
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首先我们来看一个使用条件判断的</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">Makefile</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">例子；对变量“</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">CC</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">”进行判断，其值如果是“</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">gcc</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">”那么在程序连接时使用库“</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">libgnu.so</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">”或者“</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">libgnu.a</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">”，否则不链接任何库。</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">Makefile</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">中的条件判断部分如下：</span></p>
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<span lang="EN-US" style="font-size: 12.0pt; font-family: Arial">……</span></i></b></p>
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<span lang="EN-US" style="font-size: 12.0pt; font-family: Arial">libs_for_gcc =
-lgnu</span></i></b></p>
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