How can I suppress Ant warnings for overrided properties?

I have build.xml with some global properties:

<project default="build">
  <property environment="env"/>
  <property name="root.dir" location=".."/>
  <property name="subdir.dir" location="${root.dir}/subdir"/>
  ...
</project>

My builds work fine but every time I do an <antcall>, I get a mess of override warnings for these global properties:

Override ignored for property "env.VAR1"
Override ignored for property "env.VAR2"
Override ignored for property "root.dir"
...

I get them even though I haven't actually overridden the properties in the task. The warning messages have made my ant log very verbose and hard to follow. Is there a way to suppress the warnings?

I'm using ant 1.9.4, which I can upgrade if needed.