In the 12 July build, project-level variables certainly work (I use those every day), however, the ones you set compiler-wide are not honoured.
This is not a big problem in your case, though, as the 12 July build supports GCV. Go to Settings -> Global Variables and define your variable there. In fact, this is even better because you can have different sets, so for example switching to another compiler version is very easy.
Remember that you have to reference that type of variables by $#NAME or $(#NAME), not $NAME. See the WiKi ("global compiler variables") for more info on that subject.