SET (ZERO_VAR 0)
IF(ZERO_VAR)
ADD_DEFINITIONS(-DSHOULD_NOT_BE_DEFINED)
ELSE(ZERO_VAR)
ADD_DEFINITIONS(-DSHOULD_BE_DEFINED)
ENDIF(ZERO_VAR)

SET(ONE_VAR 1)

SET(STRING_VAR "CMake is great" CACHE STRING "test a cache variable")