You are correct, I had misremembered how it works. It can evaluate build arguments, but they have to be numerical. However you can define new variables (in the pre-processor, not in code - ie, after a #) to replace those numbers, to make the intent clearer. Eg.
#define DEBUG_1 1
#define DEBUG_2 2
#if DEBUG_LEVEL >= DEBUG_1
Then pass the build arg DEBUG_LEVEL at compile time
More info here:
Edit: formatting of code snippet
It's either Nasal or Aural, and I'm actually not sure which would least worst