[CMake] When is UPDATE_COMMAND executed in External Project?

[Soonho Kong]

I'm using ExternalProject_Add in my CMakeList.txt file
<https://github.com/dreal/dreal3/blob/master/src/CMakeLists.txt#L184-L196>
and I'm
wondering when it's executed. Ideally, I want to execute this whenever a
user is running a build command (i.e. make/ninja). However, it seems that
this is not the case for now. Is there a way to achieve this goal?

Best,
Soonho
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