seems like the openexr 2.3.0 release tarball has broken cmake support, latest from git works
we'll have to revisit this once they get a new release out.
Note these are intended for platform maintainers, we do not intend to
support users making their own builds with these. For that precompiled
libraries from lib/ should be used.
Implemented by Martijn Berger, Ray Molenkamp and Brecht Van Lommel.
Differential Revision: https://developer.blender.org/D2753
