qt5.qtwebengine: fail properly

QtWebEngine's build system is setup to perform certain platform checks
(see mkspecs/features/platform.prf). But a failed check will not cause
configuration phase to fail, but instead it configures no build targets.
So in such case the build will successfully perform build and install
phases. An empty output directories will are produced and the build
succeeds.

This patches qtwebengine qmake files to properly fail during
configuration phase.

This doesn't touch qt56 as it doesn't have this mechanism.