QtWebEngine fails to build when an older Qt5 version is present in the install location

This has already been discussed on the interest ML:

Building all of Qt 5.4 in a single configure step followed by a single "make" step leads to headerfile confusion when building QtWebEngine if an older (pre 5.4) Qt version has been installed using the same "-headerdir".

It turns out that this is because qtwebengine/src/core/core_gyp_generator.pro adds that location to the headerfile search path to be used while building, apparently a left-over from Qt 5.2 days when QtWebEngine was not yet shipped with the rest of Qt.
As admitted by a comment just above the INCLUDEPATH statement:

1. something fishy with qmake in 5.2 ?

I'm attaching a patch that resolve the issue by removing QT_INSTALL_HEADERS from said INCLUDEPATH statement. This turns out to be unnecessary, at least when building with specifying just about all -*dir options.