These KINETIS defines are already in the Teensyduino core files and are automatically set, depending on which Teensy you selected in the Tools-Boards menu of the Teensyduino IDE. KINETISL is for the Teensy LC, KINETISK is for the Tennsy 3.x series and allow to dynamically compile different code variants for different hardware.
Thus, you must not define these - it will give compiler errors.
But you may query these with #if defined() in your code where you want to implement different compile variants for different Teensy models. In most cases, people don‘t need this, since they write their code for their specific hardware which has been determined in an earlier project stage. One current use case is writing a library for common use which is intended to work with different Teensy models and which has internal functions which depend on hardware specifics.