[-WSuggest-override] Add override directive if C++11 is used

[rherrmannr]

For example: Use virtual void init() override;

instead of virtual void init();

[BeckmaR]

We are not yet using or supporting C++11 in any way, so I am really not sure if this would be such a great idea... We should rather find a way to tell all compilers to keep their style warnings to themselves in our code.