Simplify CMAKE test, per review

CMakeLists.txt

@@ -9,7 +9,7 @@ SET(CMAKE_C_STANDARD 11)
 
 SET(CMAKE_C_FLAGS "-std=c99 -Wall -O2 -g ${CC_WARNING_FLAGS} ${CMAKE_C_FLAGS}")
 
-if((${DISABLE_DEBUG_PRINTF}) AND (DEFINED DISABLE_DEBUG_PRINTF))
+if(${DISABLE_DEBUG_PRINTF})
     SET(CMAKE_C_FLAGS "-DDISABLE_DEBUG_PRINTF ${CMAKE_C_FLAGS}")
 endif()