Fix CMake build error finding OpenEXR package

[PZerua]

Fixes: #467

find_package(OpenEXR) was called twice, once in the main CMakeLists.txt and another in src/ext/CMakeLists.txt. Also, when OpenEXR and ZLib are not installed in the system, trying to find the package OpenEXR in the main CMake failed with the error: "missing: ZLIB_LIBRARY ZLIB_INCLUDE_DIR" because the variables were not in the scope.

[sssfish]

I had a similar problem. Have you solved it?

[mmp]

Thanks! I believe that other PRs have taken care of this issue so closing this one out.