Provide an option that the user can optionally select.

option(<variable> "<help_text>" [value])

Provides an option for the user to select as ON or OFF. If no initial <value> is provided, OFF is used. If <variable> is already set as a normal or cache variable, then the command does nothing (see policy CMP0077).

If you have options that depend on the values of other options, see the module help for CMakeDependentOption.