* CMAKE_SHARED_LIBRARY_CXX_FLAGS is empty here, caused CMake errors.
* CMake's OBJECT target has a bug that it doesn't define target_EXPORTS
as with other library targets, so it's needed to define it manually.
It also fixes empty flags issue.
* Visibility headers weren't updated for OBJECT targets, they now also
catch *Objects_EXPORTS defines.
* For libraries that are part of the same project using directly target
name instead of ${*_LIBRARY} variable.
* Prefixed test target names with namespace, so tests for the same
class in different namespaces won't conflict.
* Removed unneeded linking to ${CORRADE_UTILITY_LIBRARY} in some
places.