COMP: Make it easier to build on Debian 9 systems
Created by: gregsharp
This makes it easier for developers to build Slicer on Debian >= 9 systems, and possibly other linux systems, which have gcc >= 6 as default.
- It allows to explicitly enable -DCMAKE_CXX_STANDARD=11 on the cmake command line instead of only allowing this based on choice of VTK/Qt version
- It allows to use system cmake <= 3.8.2 if -DSlicer_USE_SimpleITK:BOOL=OFF