[CMake] Noob question

[alpha_one_x86]

I have try this:

# Options for debug level
if(NOT DEFINED ${DEBUG_LEVEL})
  add_definitions(-DDEBUG=0)
else(NOT DEFINED ${DEBUG_LEVEL})
  add_definitions(-DDEBUG=${DEBUG_LEVEL})
endif(NOT DEFINED ${DEBUG_LEVEL})

With the command line:
cmake -DDEBUG_LEVEL=50 ./

That's not work, why?