Add note warning against use of pre-8.4 multithreaded Tcl.

doc/src/sgml/installation.sgml

-<!-- $PostgreSQL: pgsql/doc/src/sgml/installation.sgml,v 1.292 2007/08/25 20:29:25 tgl Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/installation.sgml,v 1.293 2007/10/01 16:43:28 tgl Exp $ -->
 
 <chapter id="installation">
  <title><![%standalone-include[<productname>PostgreSQL</>]]>
@@ -228,7 +228,10 @@ su - postgres
     <listitem>
      <para>
       If you want to build the <application>PL/Tcl</application>
-      procedural language, you of course need a Tcl installation.
+      procedural language, you of course need a <productname>Tcl</>
+      installation.  If you are using a pre-8.4 release of
+      <productname>Tcl</>, ensure that it was built without multithreading
+      support.
      </para>
     </listitem>