Minor improvements for CMake handling of boost/OpenEXR

A couple of minor improvements to CMake's handling of boost and OpenEXR:
* Do not overwrite the ```Boost_ADDITIONAL_VERSIONS``` variable. This allows to specify ever newer versions of boost from the CMake configuration. This could be a complement to #549.
* When not using custom library versions, find automatically IlmBase/OpenEXR when they are in the CMake default paths (including those defined by ```CMAKE_PREFIX_PATH```.) This interacts with #483.