[CMake] Manually-specified variables were not used by the project

[Haio Maio]

Manually-specified variables were not used by the project
MY_VAR

when I call 

cmake -DMY_VAR= ...


The goal is
if(NOT DEFINED MY_VAR)
...
else()
...
if()

in order to tell cmake over the command line what to do.

How to achieve this ?
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