Add description of GCC's --enable-mpfr switches.

git-svn-id: http://svn.linuxfromscratch.org/LFS/trunk/BOOK@10380 4aa44e1e-78dd-0310-a6d2-fbcd4c07a689

chapter05/gcc-pass1.xml

@@ -222,6 +222,14 @@ cd ../gcc-build</userinput></screen>
         </listitem>
       </varlistentry>
 
+      <varlistentry>
+        <term><parameter>--with-mpfr-*</parameter></term>
+        <listitem>
+          <para>These options enable the build system to correctly use the
+          in-tree copy of the MPFR sources.</para>
+        </listitem>
+      </varlistentry>
+
     </variablelist>
 
     <para>Compile GCC by running:</para>