Add CMAKE option to turn on the gui when building slic3r.cpp (defaults to off)
Add CMAKE option to turn on the gui when building slic3r.cpp (defaults to off)

lordofhyphens/Slic3r,alexrj/Slic3r,lordofhyphens/Slic3r,lordofhyphens/Slic3r,lordofhyphens/Slic3r,alexrj/Slic3r,alexrj/Slic3r,curieos/Slic3r,lordofhyphens/Slic3r,curieos/Slic3r,curieos/Slic3r,pieis2pi/Slic3r,lordofhyphens/Slic3r,pieis2pi/Slic3r,alexrj/Slic3r,curieos/Slic3r,alexrj/Slic3r,pieis2pi/Slic3r,pieis2pi/Slic3r,pieis2pi/Slic3r,pieis2pi/Slic3r,curieos/Slic3r,alexrj/Slic3r,curieos/Slic3r