ch05/libstdc++: set CXX=$LFS_TGT-gcc

In a normal GCC build the top-level Makefile automatically sets the
CXX=xgcc.  And without it the "checking for sin in -lm" probe will fail,
and then GCC will believe the math functions (for example acosf) are not
available and fail to build std.gcm (added since GCC 16) with about a
dozen lines of error messages, which would likely disconcert the user if
spotted.

chapter05/libstdc++.xml

@@ -63,6 +63,7 @@ cd       build</userinput></screen>
 <screen><userinput remap="configure">../libstdc++-v3/configure      \
     --host=$LFS_TGT            \
     --build=$(../config.guess) \
+    CXX=$LFS_TGT-gcc           \
     --prefix=/usr              \
     --disable-multilib         \
     --disable-nls              \
@@ -81,6 +82,23 @@ cd       build</userinput></screen>
         </listitem>
       </varlistentry>
 
+      <varlistentry>
+        <term><parameter>CXX=$LFS_TGT-gcc</parameter></term>
+        <listitem>
+          <para>Specifies to use <command>$LFS_TGT-gcc</command> to compile
+          the C++ code.  The default, <command>$LFS_TGT-g++</command>,
+          would automatically specify linking against
+          <systemitem class='library'>libstdc++</systemitem> when invoking
+          the linker, but in our case
+          <systemitem class='library'>libstdc++</systemitem> is not
+          installed yet so some checks in the <command>configure</command>
+          script would misbehave with <command>$LFS_TGT-g++</command>.  In a
+          normal build, this override is automatically passed to the
+          <command>configure</command> script from the top level directory.
+          Here we explicitly pass it.</para>
+        </listitem>
+      </varlistentry>
+
       <varlistentry>
         <term><parameter>--disable-libstdcxx-pch</parameter></term>
         <listitem>