I'm not sure which of QT 5.9.0 or gcc 5.5.0 is not happy, but with both
of them, I get:
```
/opt/qt59/include/QtCore/qflags.h: In instantiation of ‘QFlags<T>& QFlags<T>::setFlag(Enum, bool) [with Enum = QgsDxfExport::DxfPolylineFlag]’:
/home/even/qgis/QGIS/src/core/dxf/qgsdxfexport.cpp:1102:68: required from here
/opt/qt59/include/QtCore/qflags.h:153:46: error: no match for ‘operator~’ (operand type is ‘QgsDxfExport::DxfPolylineFlag’)
return on ? (*this |= f) : (*this &= ~f);
```