Only link to Imath when OpenEXR version is >= 3.0
Since OpenEXR 3.0, Imath is in a separate, independently released library. Since that's the only library that Calligra needs, there's no need to link to the full OpenEXR
Since OpenEXR 3.0, Imath is in a separate, independently released library. Since that's the only library that Calligra needs, there's no need to link to the full OpenEXR