CONFIG += c++1z works, CONFIG += c++17 doesn't (erratic)

Compiling C++17 code (Qt-Creator4.6 and Qt-5.10.1)
 
Inserting 'CONFIG += c++1z' in .pro file works fine.
'CONFIG += c++17' seems to work, but then behaves erratic, as an additional '-std=gnu++11' is being added to the make options.
It did cost me half an hour to figure this out. ;-|