UNSET NOTE

.. 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.