[CMake] COMPILE_DEFINITIONS

[John Drescher]

My goal is to a specific definition to the Debug and RelWithDebInfo
configurations for all targets in the project but not to the Release
configuration when using VisualStudio. I believe that
COMPILE_DEFINITIONS<CONFIG> appears to support this however I am
unsure if I have to get the previous value of the COMPILE_DEFINITIONS
before setting the new value?

Thanks,
John M. Drescher