Add rationale for Glib and PCRE. Fixes #2885.

git-svn-id: http://svn.linuxfromscratch.org/LFS/trunk/BOOK@9561 4aa44e1e-78dd-0310-a6d2-fbcd4c07a689

prologue/why.xml

@@ -162,6 +162,14 @@
         internationalization and localization of numerous packages.</para>
       </listitem>
 
+      <listitem>
+        <para>Glib</para>
+
+        <para>This package contains low-level core library functions for things
+        such as event loops and threading.  It is used by one other LFS package,
+        Pkg-config.</para>
+      </listitem>
+
       <listitem>
         <para>Glibc</para>
 
@@ -328,6 +336,15 @@
         the build procedure for several LFS packages.</para>
       </listitem>
 
+      <listitem>
+        <para>PCRE</para>
+
+        <para>This package is provides a library that can be used to implement
+        regular expression pattern matching using the same syntax and semantics
+        as Perl 5.  It is a mandatory dependency of Glib, and can also be used
+        by Grep.</para>
+      </listitem>
+
       <listitem>
         <para>Perl</para>