[Cmake] How to remove a #define?

[Mathews, Rob]

The CMake documentation says that you do this

ADD_DEFINITIONS - Add -D define flags to command line for environments.  
Usage: ADD_DEFINITIONS(-DFOO -DBAR ...) Add -D define flags to command line
for environments. 

to define a variable for the preprocessor. I'm wondering how you do the
reverse - how to undefine an variable for the preprocessor.

FYI, I've got an example where I want 
a default of "UNICODE" defined for all the projects, except
in this particular subdir, where I don't want UNICODE
defined. 

My environment is .NET 2003.

Is there any way to do this?