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can't tell it for sure, but try installing qtchooser and setting the QT_SELECT environment variable accordingly:

but wait, you're using a non-distro packaged Qt5 installation, right? probably you got it directly from http://qt.io and, i suppose, it's installed under /opt/Qt/5.8. now, i guess, you can try the build again, just make sure you get the configure option right "--with-qt5=/opt/Qt/5.8" might not be enough! as far as i can tell, it should be more explicit on the compiler platform you're into, like "--with-qt5=/opt/Qt/5.8/gcc_64"... could it be the case?

however, if you have another Qt5 installation around, possibly the official distro-supplied one, then you'll probbably need to make up with a new custom symlink for qmake-qt5 so that ./configure --with-qt5=/opt/Qt/5.8/gcc_64 will get the right configuration. please, do only this if you're sure there's yet another Qt5 installation out there:

hth.
cheers