Add config.guess update to plan.

misc/plan.html

@@ -105,6 +105,9 @@
   </p>
   
   <h2> Build system </h2>
+  <p class='should'>
+    Update config.guess and config.sub. Needed for ppc64le support.    
+  </p>
   <p class='should'>
     Stop using the nonstandard <tt>.po</tt> extension,
     using <tt>.p.o</tt> or some subdirectory instead. Also drop