Cmake build: Added code to compile with QT5.7

Cmake build: Added code to compile with QT5.7

QT5.7 cmake config files explicitly instruct cmake to use `-std=gnu++11`.
This is not the same as is found by our FindCXX11Compiler.cmake,
which leads to compilation errors.
Here we overrule the setting.

Thanks to helpfull hand from Brad King <brad.king@kitware.com>