Ninja for qtwebengine out-of-source build not possible

When I do an out-of-source build, e.g. sources in 'qt5' and a
'../qt5/configure' in a separate 'qt5-build' folder, ninja for the qtwebengine is
still built within qt5/qtwebengine/src/3rdparty/ninja/ and not in the qt5-build tree.

Apart from the pure cosmetic ugliness to have created code in a folder
below 'src', it is ugly because it taints the sources, which prevents
to move the source tree easily to a different machine, or have the
sources read-only.