groovyc in 1.6.0 no longer supports 'deprecation' attribute on nested javac

With Groovy 1.5.1 I used the 'deprecation' attribute on <javac> nested within the <groovyc> Ant task to have the compiler spit out deprecated API warnings:

<groovyc ...>
...
<javac deprecation="${javac.deprecation}" .../>
</groovyc>

I'm trying to upgrade to Groovy 1.6.0 but it complains that 'deprecation' is no longer a supported attribute on <javac> (within <groovyc> that is - it still works fine from a regular <javac> call). If I invoke groovyc from AntBuilder (within a <groovy> element in my Ant script), the error goes away but the attribute has no effect. I can't find any way to get my deprecation warnings back.