gettext: Fix build issue with system libxml2

The upstream fix is explicitly checking libxml2 version so it's not
practical to be turned into a sed.  But I don't want to upload another
patch just for such a "temporary" issue (as it'll be fixed for 0.23.1
anyway).  This alternative fix relies on:

1. The fact that the return type of xmlCtxtGetLastError and the arg type
   of xmlStructuredErrorFunc are changed in the same commit
   61034116d0a3 ("error: Make more xmlError structs constant").
2. GCC supports typeof(expr) to get the type of expr w/o really
   evaluating it.

So it should have the same effect as the upstream fix.

Tested with both --with-included-libxml2 and --without-included-libxml2
on BLFS.

chapter08/gettext.xml

@@ -43,6 +43,14 @@
   <sect2 role="installation">
     <title>Installation of Gettext</title>
 
+    <para>At first, fix an issue causing the package fail to build with
+    libxml-2.12 or later.  The fix is optional for building LFS, but
+    required if rebuilding this package in BLFS with libxml
+    installed:</para>
+
+<screen><userinput remap="pre">sed -e '/^structured/s/xmlError \*/typeof(xmlCtxtGetLastError(NULL)) /' \
+    -i gettext-tools/src/its.c</userinput></screen>
+
     <para>Prepare Gettext for compilation:</para>
 
 <screen><userinput remap="configure">./configure --prefix=/usr    \