configure tries and fails to write config.log to /

Check out qt-5.8 and run configure.
I used the switches -developer-build -opensource -nomake examples -nomake tests -confirm-license in a shadow build directory.
You'll get many error message like:

/home/qtrob/dev/src/qt-5.8.0-pre/qtbase/mkspecs/features/configure_base.prf:28: Cannot write file /config.log: Permission denied

Full output attached.

Configure should not try writing to / but use a location inside the build directory instead.