Building QPA results in Project ERROR: Could not find feature reduce_exports.

I have a two projects, a QPA extending eglfs, and an eglfs device integration. Both are plugins using the load(qt_plugin) command in the respective .pro files.

Both projects build and run fine on Qt 5.6.0. Migrating to Qt 5.8.0 I get this when executing make either from within Qt Creator or directly on the console.

"/opt/qt/5.8.0/linux-arm-poky-gnueabi-g++/qtbase/bin/qmake" /home/naim/Documents/naim/projects/leo/svn/trunk/Source/UI/Chalawan/deviceintegration/eglfs_sgx/eglfs_sgx.pro -r -spec linux-arm-gnueabi-g++ CONFIG+=debug CONFIG+=qml_debug
Info: creating stash file /media/psf/Home/Documents/naim/projects/leo/svn/trunk/Source/UI/.qmake.stash
Project ERROR: Could not find feature reduce_exports.

Using the broadcom eglfs device integration provided with Qt I can reproduce the error by moving that project out of the Qt source tree and into my SVN repository working area. Suggests something new in 5.8.0 is reliant on plugins being built from inside an expected Qt source folder perhaps.