Fix cmake OpenEXR 3.0

The new OpenEXR release, now mainstream in Arch Linux, breaks the compilation of ART:

We shall rely on the own OpenEXR cmake module now shipped when using OpenEXR 3.0 but still be backward compatible with OpenEXR 2.0.

Also, some include path have changed.

This branch is made to fix these different issues and have a common code base for both OpenEXR 2.0 & 3.0

https://github.com/AcademySoftwareFoundation/Imath/blob/master/docs/PortingGuide2-3.md