Use pkgconfig in FindOpenEXR.cmake if available (#2008)
Use pkgconfig in FindOpenEXR.cmake if available (#2008)

* added search for IlmBase and OpenEXR with pkgconfig if found

* Added _ILMBASE/OPENEXR_LIBDIR variables found by pkgconfig to GENERIC_LIBRARY_PATHS

OpenImageIO/oiio,lgritz/oiio,lgritz/oiio,lgritz/oiio,OpenImageIO/oiio,OpenImageIO/oiio,lgritz/oiio,OpenImageIO/oiio