[CMake] XCode: Detect debug build to enable /D_Debug or similar?

[g4 at novadsp.com]

How can I do this? Specifically generating for XCode.

My cmakefile has this:

if (${CMAKE_CFG_INTDIR} STREQUAL "Debug")
Message("We are generating/building debug code for ${this_target}")
ADD_DEFINITIONS(/D_DEBUG)
endif()

But _DEBUG is never defined.

This question comes up twice on StackOverflow. Both answers are 
endlessly mutually referential ...

Thx++