Can not use C++11 in Qt

Since Qt 5.7, C++11 support is mandatory. For some reason one large old project does not accept the C++11 arguments. It doesn't matter if I use "CONFIG += c++11" or any other; C++11 only features do not work and therefor I can not compile the project.

The compiler arguments do contain "-std=c++11 -std=c++0x" if I use "CONFIG += c++11" in the .pro file. The target platform is Android ArmV7.

Any idea is welcome :)