* 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.
Reusing macros MAGNUM_EXPORT and friends already used for Windows
builds. For exported classes added MAGNUM_LOCAL to private members which
are not referenced from any inline function. Added explicit
MAGNUM_EXPORT to private members which are referenced. CMake provides
its own macro <target>_EXPORTS, using that instead of
set_target_properties().