The fix for
QTBUG-70056 (2aa779e893bd7526c97a1968c6835204e4e985e2) breaks building wayland extension, if the source dir of the module is not on the same "depth" level in the filesystem as the qtbase source dir.
Qt build from source in ~/git/qt (standard dev shadow build from git into ~/git/build-qt)
Building QtWayland together with Qt (so its sources are in ~/git/qt/qtwayland) works ok.
BUT, moving QtWayland out to e.g. ~/foo/qtwayland (and shadow building into ~/foo/build-qtwayland) breaks the build, because the generated forward include header tries to include the generated Wayland extension header using a relative path that has one "../" to much.
The funny thing is now that moving QtWayland to ~/foo/bar/qtwayland works again, because the amount of "../" needed to jump from qtbase's build dir to qtwayland's source dir is the same.
The same problem applies to any module using Wayland extension (e.g. QtApplicationManager), but it is easily reproduced with QtWayland - that's why I chose it as an example here.