If a user manually changes a generated file, don't ever overwrite it.

That way the user can modify an auto-generated 'compile' script, for
example, and it'll stay modified.

If they delete the file, we can then generate it for them again.

Also, we have to warn whenever we're doing this, or people might think it's
a bug.

redo-ifchange.py

@@ -7,6 +7,9 @@ from helpers import debug, debug2, err, unlink
 def dirty_deps(f, depth, max_changed):
     if vars.DEBUG >= 1: debug('%s?%s\n' % (depth, f.name))
 
+    if f.failed_runid:
+        debug('%s-- DIRTY (failed last time)\n' % depth)
+        return True
     if f.changed_runid == None:
         debug('%s-- DIRTY (never built)\n' % depth)
         return True
@@ -35,6 +38,8 @@ def dirty_deps(f, depth, max_changed):
                           max_changed = f.changed_runid):
                 debug('%s-- DIRTY (sub)\n' % depth)
                 return True
+    if f.is_override:
+        builder.warn_override(f.name)
     f.set_checked()
     f.save()
     return False