neither webengine nor WebEngine substrings appear in qt-build/config.log.
Before reaching out on forum, I did try the following:
cd qt-build (at same level as qt-everywhere-src-5.10.0)
mkdir qtwebengine
cd qtwebengine
../qtbase/qmake ../qt-everywhere-src-5.10.0/qtwebengine
it said that there was no file in qt-build/qtwebengine named qtwebengine-config.pri. That is correct; nothing is in qt-build-qtwebengine at the point of doing the qmake. as it happens, there is a file called cofigure.pri inside of the source directory for qtwebengine, but not one called qtwebengine-config.pri