Error while Cross-Compiling QTWebEngine for RPi3

[martink]

[0_1549108780498_qmake_log.zip](Uploading 100%) [Hello guys.
I try to compile a Qt System (5.10) for EGLFS on RaspberryPI 3 for which I followed the guide at https://wiki.qt.io/RaspberryPi2EGLFS
Basically what I want to have is Qtbase + Qtwebengine + QtWebBrowser. It should run fullscreen without X11 in the end.

Compiling Qtbase worked very well on my host which is a Ubuntu 18.04.1 LTS (x64) running in a VM.
My problem however is, that I stuck on configuration of Qtwebengine with the following error:
"Project ERROR: Unknown feature object system-zlib in expression 'config.unix && features.system-zlib && libs.webengine-zlib'." And it seems it cannot find some libraries of which I know they are installed on the PI and synced to the host (like libpng for example):

~/raspi/qt5/bin/qmake -r CONFIG+=release
Info: creating stash file /home/martin/qtwebengine/.qmake.stash
Info: creating cache file /home/martin/qtwebengine/.qmake.cache

Running configuration tests...
Checking for alsa... yes
Checking for bison... yes
Checking for embedded... yes
Checking for flex... yes
Checking for gperf... yes
Checking for host pkg-config... /usr/bin/pkg-config
Checking for pulseaudio >= 0.9.10... no
Checking for python2... /usr/bin/python2
Checking for d-bus... no
Checking for fontconfig... no
Checking for glib-2.0 >= 2.32.0... no
Checking for glibc > 2.16... yes
Checking for harfbuzz >= 1.4.2... no
Checking for jsoncpp... no
Checking for khr... yes
Checking for libdrm... no
Checking for libevent... no
Checking for srtp... no
Checking for libvpx... no
Checking for libwebp, libwebpmux and libwebpdemux... no
Checking for libxml2 and libxslt... no
Checking for minizip... no
Checking for system ninja... yes
Checking for nss... no
Checking for opus... no
Checking for libpng >= 1.6.0... no
Checking for protobuf... no
Checking for re2... no
Checking for snappy... no
Checking for xcomposite... no
Checking for xcursor... no
Checking for xi... no
Checking for xrandr... no
Checking for xtst... no
Project ERROR: Unknown feature object system-zlib in expression 'config.unix && features.system-zlib && libs.webengine-zlib'.

I have installed any build dependecies according to https://wiki.qt.io/QtWebEngine/How_to_Try
I have tried to get some useful information out of the debug log from qmake, but this wasn't very helpful for me.
Here are the qmake persistent variables (In case they may provide some useful information to you):

~/raspi/qt5/bin/qmake -query
QT_SYSROOT:/home/martin/raspi/sysroot
QT_INSTALL_PREFIX:/home/martin/raspi/qt5pi
QT_INSTALL_PREFIX/dev:/usr/local/qt5pi
QT_INSTALL_ARCHDATA:/home/martin/raspi/qt5pi
QT_INSTALL_ARCHDATA/dev:/usr/local/qt5pi
QT_INSTALL_DATA:/home/martin/raspi/qt5pi
QT_INSTALL_DATA/dev:/usr/local/qt5pi
QT_INSTALL_DOCS:/home/martin/raspi/qt5pi/doc
QT_INSTALL_DOCS/dev:/usr/local/qt5pi/doc
QT_INSTALL_HEADERS:/home/martin/raspi/qt5pi/include
QT_INSTALL_HEADERS/dev:/usr/local/qt5pi/include
QT_INSTALL_LIBS:/home/martin/raspi/qt5pi/lib
QT_INSTALL_LIBS/dev:/usr/local/qt5pi/lib
QT_INSTALL_LIBEXECS:/home/martin/raspi/qt5pi/libexec
QT_INSTALL_LIBEXECS/dev:/usr/local/qt5pi/libexec
QT_INSTALL_BINS:/home/martin/raspi/qt5pi/bin
QT_INSTALL_BINS/dev:/usr/local/qt5pi/bin
QT_INSTALL_TESTS:/home/martin/raspi/qt5pi/tests
QT_INSTALL_TESTS/dev:/usr/local/qt5pi/tests
QT_INSTALL_PLUGINS:/home/martin/raspi/qt5pi/plugins
QT_INSTALL_PLUGINS/dev:/usr/local/qt5pi/plugins
QT_INSTALL_IMPORTS:/home/martin/raspi/qt5pi/imports
QT_INSTALL_IMPORTS/dev:/usr/local/qt5pi/imports
QT_INSTALL_QML:/home/martin/raspi/qt5pi/qml
QT_INSTALL_QML/dev:/usr/local/qt5pi/qml
QT_INSTALL_TRANSLATIONS:/home/martin/raspi/qt5pi/translations
QT_INSTALL_TRANSLATIONS/dev:/usr/local/qt5pi/translations
QT_INSTALL_CONFIGURATION:/home/martin/raspi/qt5pi/etc/xdg
QT_INSTALL_CONFIGURATION/dev:/usr/local/qt5pi/etc/xdg
QT_INSTALL_EXAMPLES:/home/martin/raspi/qt5pi/examples
QT_INSTALL_EXAMPLES/dev:/usr/local/qt5pi/examples
QT_INSTALL_DEMOS:/home/martin/raspi/qt5pi/examples
QT_INSTALL_DEMOS/dev:/usr/local/qt5pi/examples
QT_HOST_PREFIX:/home/martin/raspi/qt5
QT_HOST_DATA:/home/martin/raspi/qt5
QT_HOST_BINS:/home/martin/raspi/qt5/bin
QT_HOST_LIBS:/home/martin/raspi/qt5/lib
QMAKE_SPEC:linux-g++
QMAKE_XSPEC:devices/linux-rasp-pi3-g++
QMAKE_VERSION:3.1
QT_VERSION:5.10.1

Seems like there was a related bug a some years ago when building on windows:
https://codereview.qt-project.org/#/c/206419
But, it looks like this has been fixed and I don't think it would apply here.

Does anybody have a hint what I can try to solve that problem?
Do you need more info about the build environment?

Best regards
Martin.