When building QtWebEngine 5.15.0 with Qt 5.9, I get several errors like the following:
error: 'endl' is not a member of 'Qt'
(recent change: https://github.com/qt/qtwebengine/commit/a2feb993aea9ab39d823daaa5d983546f0dd758d#diff-4a4b27fe3adcf64d068ea42ae0c97fc5 which converted endl to Qt::endl)
I suggest keeping the possibility to build QtWebEngine with earlier still supported Qt versions, since the version of QtWebEngine can be upgraded without upgrading the whole Qt framework version.
e.g. by adding https://pastebin.com/5XbSBihH