redo.py: report when you're trying to rebuild a static file.

In redo-ifchange, this might be a good idea, since you might just want to
set a dependency on it, so we won't say anything from inside builder.py.
But if you're calling redo.py, that means you expect it to be rebuilt, since
there's no other reason to try.  So print a warning.

(This is what make does, more or less.)

redo.py

@@ -39,9 +39,16 @@ import vars_init
 vars_init.init(targets)
 
 import vars, state, builder, jwack
-from log import err
+from log import warn, err
 
 try:
+    for t in targets:
+        if os.path.exists(t):
+            f = state.File(name=t)
+            if not f.is_generated:
+                warn('%s: exists and not marked as generated; not redoing.\n'
+                     % f.nicename())
+    
     j = atoi(opt.jobs or 1)
     if j < 1 or j > 1000:
         err('invalid --jobs value: %r\n' % opt.jobs)