| .. note:: | |
| When evaluating :ref:`Variable References` of the form ``${VAR}``, CMake | |
| first searches for a normal variable with that name. If no such normal | |
| variable exists, CMake will then search for a cache entry with that name. | |
| Because of this, **unsetting a normal variable can expose a cache variable | |
| that was previously hidden**. To force a variable reference of the form | |
| ``${VAR}`` to return an empty string, use ``set(<variable> "")``, which | |
| clears the normal variable but leaves it defined. |