CMakeLists.txt - for OpenEXR3 make sure IMath has been found

Ran into this when trying to build ART on a minimal Ubuntu 22.04 LTS on Windows WSL. With OpenEXR 3 Imath has been split into a separate project. We need not just look for it via find_package, we also need to make sure it has been found.

Is there some aumation that makes sure this not break the build for OpenEXR 3 and 2.x?

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• 572d281d - CMakeLists.txt - use correct flag to check if OpenEXR 3 has been found

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