A few more fixes for undo file. Split test in two parts so that it doesn't

fail with tiny features.

runtime/doc/todo.txt

@@ -1093,6 +1093,7 @@ Vim 7.3:
    Use register_shell_extension()? (George Reilly, 2010 May 26)
    Ron's version: http://dev.ronware.org/p/vim/finfo?name=gvim.nsi
 - Persistent undo bugs / fixes:
+    - check for sizeof(time_t) results in compiler warning in misc2.c
     - Memory leak reproduced by Dominique Pelle, 2010 May 28.
     - When there is no undo info (undolevels negative), delete the undo file.
     - Need to check all values for evil manipulation.