Update description/rationale for Libstdc++ in Chapter 5

was r11424.

chapter05/gcc-pass1.xml

@@ -5,7 +5,7 @@
   %general-entities;
 ]>
 
-<sect1 id="ch-tools-gcc-pass1" role="wrap">
+<sect1 id="ch-tools-gcc-pass1" role="wrap" xreflabel="第一遍的 GCC">
   <?dbhtml filename="gcc-pass1.html"?>
 
   <sect1info condition="script">

chapter05/libstdc++.xml

@@ -14,7 +14,7 @@
     <address>&gcc-url;</address>
   </sect1info>
 
-  <title>Libstdc++-&gcc-version;</title>
+  <title>GCC-&gcc-version; 中的 Libstdc++</title>
 
   <indexterm zone="ch-tools-libstdcpp">
     <primary sortas="a-GCC">GCC</primary>
@@ -24,9 +24,16 @@
   <sect2 role="package">
     <title/>
 
-    <!--para>Libstdc++ is the standard C++ library. It is needed for the correct
-    operation of the g++ compiler.</para-->
-    <para>Libstdc++ 是 C++ 标准库，g++ 编译器的正常工作需要它。</para>
+    <!--para>Libstdc++ is the standard C++ library. It is needed 
+    to compile C++ code
+    (part of GCC is written in C++), but we had to defer its installation
+    when we built <xref linkend="ch-tools-gcc-pass1"/>
+    because it depends on glibc, which was not yet available in /tools.
+    </para-->
+	<para>Libstdc++ 是 C++ 标准库，我们需要它才能编译 C++ 代码
+    （GCC 的一部分用 C++ 编写）。但在构建
+	<xref linkend="ch-tools-gcc-pass1"/> 时我们不得不暂缓安装它，
+	因为它依赖于当时还没有安装到 /tools 的 glibc。</para>
 
     <segmentedlist>
       <segtitle>&buildtime;</segtitle>