[Boost-build] How to write a warn-once rule?

Subject: [Boost-build] How to write a warn-once rule?
From: Matthew Chambers (matt.chambers42_at_[hidden])
Date: 2013-02-11 17:47:53

I'm trying to write a rule like:

rule warn-once ( once-flag ? : message )
{
     if ! $(once-flag)
     {
        once-flag = true ;
         echo $(message) ;
     }
}

But it doesn't work because once-flag is passed by value. Is there a way to pass it by reference or 
a different way to approach this while still getting a simple interface?

Thanks,
-Matt