geant/gexace question

I need a finer switch in my library.xace files then the "windows/unix"
GOBO_OS variable.

I have code like the following in my .xace files:
   <option unless="${ARCH}=macosx-ppc">
where ARCH is defined in the environment. The paths/libraries/library
flags change with the different architectures I support. Obviously,
this doesn't work, the code is always executed no matter what ${ARCH}
is set to.

So, how do I make this work, or what is the right way to achieve this
effect?

Thanks,

Mark