Implement proper C++ visibility support?

[claudiofantacci]

Implement C++ visibility support as described here.

This will probably allow us to remove this Windows-specific CMake instruction.

An example of this implementation can be found in YARP in conf/api.h used, e.g., in os/api.h.

[claudiofantacci]

See discussion in robotology/superimpose-mesh-lib#15