Document new -with-libxslt build option.

doc/src/sgml/installation.sgml

-<!-- $PostgreSQL: pgsql/doc/src/sgml/installation.sgml,v 1.286 2007/03/13 16:03:36 mha Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/installation.sgml,v 1.287 2007/04/21 15:30:28 adunstan Exp $ -->
 
 <chapter id="installation">
  <title><![%standalone-include[<productname>PostgreSQL</>]]>
@@ -933,6 +933,17 @@ su - postgres
        </listitem>
       </varlistentry>
 
+      <varlistentry>
+       <term><option>--with-libxslt</option></term>
+       <listitem>
+        <para>
+         Build with libxslt. The <filename>contrib/xml2</> module 
+		 relies on this library to perform Stylesheet transformations of
+		 XML. 
+        </para>
+       </listitem>
+      </varlistentry>
+
       <varlistentry>
        <term><option>--enable-integer-datetimes</option></term>
        <listitem>