Variable assignation in a target

[Gauvain Pocentek]

Hi,

I try to define a variable this way:

-----
config.status-%:
   DEB_BUILDDIR_$* ?= build-$*
----

But it doesn't work, DEB_BUILDDIR_$* is understood as a command by make.
How could I define this variable?
Should I define it outside a target? I tried this as well, but can't 
seem to make it work.

Thanks,
Gauvain