Select modules at configure time

[christiana]

The entire library now requires OpenIGTLInk, VTK, Qt, CTK. It would be nice to be able to turn the GUI parts on/off in cmake, possibly others modules as well.

Components that could be optional:

• Devices/Logic
• GUI
• Examples

[drouin-simon]

As far as I can tell, dependency on is only there for GUI. It should remain like this. Once we implement build options, it will be possible to create a client without CTK by simply disabling GUI.

[christiana]

Yes, this applies to both Qt and CTK.