COMP: Require C++11 support for building Slicer
By requiring compiler support for C++11 we can greatly simplify the code updates necessary support ITKv5 updates.
As development of the next phase of Slicer is begining, changing to require C++11 will greatly reduce the number of compile time conditionals in the code. In many cases supporting ITKv5 involves using c++11 features directly without needing conditional wrappers.