SUID executable doesn't work for `/dev/mem` access

Question

Here is explained how to gain access to the peripheral without being root. I've just tried in my environment (Raspbian Stretch, RPi2, Qt5.9) in this way:

int main(int argc, char *argv[])
{
    QCoreApplication::setSetuidAllowed(true);
    QCoreApplication a(argc, argv);
    MyServer server;   
    return a.exec();
}

then, on the target:

$ sudo chown root:root MyServer
$ sudo chmod u+s MyServer

but running it as normal user leads to a segmentation fault. Here's the output of gdb:

$ gdb MyServer 
...
run(gdb) run
Starting program: /home/pi/bin/MyServer 
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/arm-linux-gnueabihf/libthread_db.so.1".
bcm2835_init: Unable to open /dev/mem: Operation not permitted
...

Am I missing some steps?