Build issue when cross-compiling Qt with bundled xcb

I am cross building qt on LibreElec (Linux) for x86.
We use X11/EGLFS integration there.
5.7.0 was building fine but 5.8.0 Beta gives me the following build error:

make[6]: Leaving directory '/home/longchair/LibreELEC.tv/build.PlexMediaPlayer-Generic.x86_64-8.0-devel/qt5-5.8.0-beta/qtbase/src/plugins/platforms/linuxfb'
/home/longchair/LibreELEC.tv/build.PlexMediaPlayer-Generic.x86_64-8.0-devel/toolchain/bin/x86_64-libreelec-linux-gnu-g++ -c -pipe -m64 -fPIC --sysroot=/home/longchair/LibreELEC.tv/build.PlexMediaPlayer-Generic.x86_64-8.0-devel/toolchain/x86_64-libreelec-linux-gnu/sysroot -O2 -g -std=c+11 -fvisibility=hidden -fvisibility-inlines-hidden -fno-exceptions -Wall -W -Wvla -Wdate-time -D_REENTRANT -fPIC -DQT_NO_FOREACH -DQT_BUILD_XCB_PLUGIN -DXCB_USE_XLIB -DXCB_USE_XINPUT2 -DXCB_USE_RENDER -DDFLT_XKB_CONFIG_ROOT=\"/usr/share/X11/xkb\" -DDEFAULT_XKB_RULES=\"evdev\" -DDEFAULT_XKB_MODEL=\"pc105\" -DDEFAULT_XKB_LAYOUT=\"us\" -DQT_BUILD_XCB_QPA_LIB_LIB -DQT_BUILDING_QT -DQT_NO_CAST_TO_ASCII -DQT_ASCII_CAST_WARNINGS -DQT_MOC_COMPAT -DQT_USE_QSTRINGBUILDER -DQT_DEPRECATED_WARNINGS -DQT_DISABLE_DEPRECATED_BEFORE=0x050000 -DQT_NO_EXCEPTIONS -D_LARGEFILE64_SOURCE -D_LARGEFILE_SOURCE -DQT_NO_DEBUG -DQT_SERVICE_SUPPORT_LIB -DQT_THEME_SUPPORT_LIB -DQT_EVENTDISPATCHER_SUPPORT_LIB -DQT_FONTDATABASE_SUPPORT_LIB -DQT_GUI_LIB -DQT_CORE_LIB -I. -Igl_integrations -I../../../3rdparty/xkbcommon -I../../../3rdparty/xkbcommon/xkbcommon -I../../../3rdparty/xkbcommon/src -I../../../3rdparty/xkbcommon/src/xkbcomp -I../../../../include -I../../../../include/QtServiceSupport -I../../../../include/QtServiceSupport/5.8.0 -I../../../../include/QtServiceSupport/5.8.0/QtServiceSupport -I../../../../include/QtThemeSupport -I../../../../include/QtThemeSupport/5.8.0 -I../../../../include/QtThemeSupport/5.8.0/QtThemeSupport -I../../../../include/QtEventDispatcherSupport -I../../../../include/QtEventDispatcherSupport/5.8.0 -I../../../../include/QtEventDispatcherSupport/5.8.0/QtEventDispatcherSupport -I../../../../include/QtFontDatabaseSupport -I../../../../include/QtFontDatabaseSupport/5.8.0 -I../../../../include/QtFontDatabaseSupport/5.8.0/QtFontDatabaseSupport -I../../../../include/QtGui/5.8.0 -I../../../../include/QtGui/5.8.0/QtGui -I../../../../include/QtGui -I../../../../include/QtCore/5.8.0 -I../../../../include/QtCore/5.8.0/QtCore -I../../../../include/QtCore -I.moc -isystem /home/longchair/LibreELEC.tv/build.PlexMediaPlayer-Generic.x86_64-8.0-devel/toolchain/x86_64-libreelec-linux-gnu/sysroot/usr/include/libdrm -I../../../../3rdparty/xcb/include -I../../../../3rdparty/xcb/sysinclude -isystem /home/longchair/LibreELEC.tv/build.PlexMediaPlayer-Generic.x86_64-8.0-devel/toolchain/x86_64-libreelec-linux-gnu/sysroot/usr/include/freetype2 -I/home/longchair/LibreELEC.tv/build.PlexMediaPlayer-Generic.x86_64-8.0-devel/toolchain/x86_64-libreelec-linux-gnu/sysroot/home/longchair/LibreELEC.tv/build.PlexMediaPlayer-Generic.x86_64-8.0-devel/toolchain/x86_64-libreelec-linux-gnu/sysroot/usr/include/freetype2 -isystem /home/longchair/LibreELEC.tv/build.PlexMediaPlayer-Generic.x86_64-8.0-devel/toolchain/x86_64-libreelec-linux-gnu/sysroot/usr/include/freetype2 -I../../../../mkspecs/devices/linux-Generic-g+ -o .obj/qxcbscreen.o qxcbscreen.cpp
/home/longchair/LibreELEC.tv/build.PlexMediaPlayer-Generic.x86_64-8.0-devel/toolchain/bin/x86_64-libreelec-linux-gnu-g++ -c -pipe -m64 -fPIC --sysroot=/home/longchair/LibreELEC.tv/build.PlexMediaPlayer-Generic.x86_64-8.0-devel/toolchain/x86_64-libreelec-linux-gnu/sysroot -O2 -g -std=c+11 -fvisibility=hidden -fvisibility-inlines-hidden -fno-exceptions -Wall -W -Wvla -Wdate-time -D_REENTRANT -fPIC -DQT_NO_FOREACH -DQT_BUILD_XCB_PLUGIN -DXCB_USE_XLIB -DXCB_USE_XINPUT2 -DXCB_USE_RENDER -DDFLT_XKB_CONFIG_ROOT=\"/usr/share/X11/xkb\" -DDEFAULT_XKB_RULES=\"evdev\" -DDEFAULT_XKB_MODEL=\"pc105\" -DDEFAULT_XKB_LAYOUT=\"us\" -DQT_BUILD_XCB_QPA_LIB_LIB -DQT_BUILDING_QT -DQT_NO_CAST_TO_ASCII -DQT_ASCII_CAST_WARNINGS -DQT_MOC_COMPAT -DQT_USE_QSTRINGBUILDER -DQT_DEPRECATED_WARNINGS -DQT_DISABLE_DEPRECATED_BEFORE=0x050000 -DQT_NO_EXCEPTIONS -D_LARGEFILE64_SOURCE -D_LARGEFILE_SOURCE -DQT_NO_DEBUG -DQT_SERVICE_SUPPORT_LIB -DQT_THEME_SUPPORT_LIB -DQT_EVENTDISPATCHER_SUPPORT_LIB -DQT_FONTDATABASE_SUPPORT_LIB -DQT_GUI_LIB -DQT_CORE_LIB -I. -Igl_integrations -I../../../3rdparty/xkbcommon -I../../../3rdparty/xkbcommon/xkbcommon -I../../../3rdparty/xkbcommon/src -I../../../3rdparty/xkbcommon/src/xkbcomp -I../../../../include -I../../../../include/QtServiceSupport -I../../../../include/QtServiceSupport/5.8.0 -I../../../../include/QtServiceSupport/5.8.0/QtServiceSupport -I../../../../include/QtThemeSupport -I../../../../include/QtThemeSupport/5.8.0 -I../../../../include/QtThemeSupport/5.8.0/QtThemeSupport -I../../../../include/QtEventDispatcherSupport -I../../../../include/QtEventDispatcherSupport/5.8.0 -I../../../../include/QtEventDispatcherSupport/5.8.0/QtEventDispatcherSupport -I../../../../include/QtFontDatabaseSupport -I../../../../include/QtFontDatabaseSupport/5.8.0 -I../../../../include/QtFontDatabaseSupport/5.8.0/QtFontDatabaseSupport -I../../../../include/QtGui/5.8.0 -I../../../../include/QtGui/5.8.0/QtGui -I../../../../include/QtGui -I../../../../include/QtCore/5.8.0 -I../../../../include/QtCore/5.8.0/QtCore -I../../../../include/QtCore -I.moc -isystem /home/longchair/LibreELEC.tv/build.PlexMediaPlayer-Generic.x86_64-8.0-devel/toolchain/x86_64-libreelec-linux-gnu/sysroot/usr/include/libdrm -I../../../../3rdparty/xcb/include -I../../../../3rdparty/xcb/sysinclude -isystem /home/longchair/LibreELEC.tv/build.PlexMediaPlayer-Generic.x86_64-8.0-devel/toolchain/x86_64-libreelec-linux-gnu/sysroot/usr/include/freetype2 -I/home/longchair/LibreELEC.tv/build.PlexMediaPlayer-Generic.x86_64-8.0-devel/toolchain/x86_64-libreelec-linux-gnu/sysroot/home/longchair/LibreELEC.tv/build.PlexMediaPlayer-Generic.x86_64-8.0-devel/toolchain/x86_64-libreelec-linux-gnu/sysroot/usr/include/freetype2 -isystem /home/longchair/LibreELEC.tv/build.PlexMediaPlayer-Generic.x86_64-8.0-devel/toolchain/x86_64-libreelec-linux-gnu/sysroot/usr/include/freetype2 -I../../../../mkspecs/devices/linux-Generic-g+ -o .obj/qxcbwindow.o qxcbwindow.cpp
In file included from qxcbkeyboard.cpp:39:0:
qxcbkeyboard.h:45:29: fatal error: xcb/xcb_keysyms.h: No such file or directory
compilation terminated.
ln -s libQt5EglFSDeviceIntegration.so.5.8.0 libQt5EglFSDeviceIntegration.so
ln -s libQt5EglFSDeviceIntegration.so.5.8.0 libQt5EglFSDeviceIntegration.so.5
ln -s libQt5EglFSDeviceIntegration.so.5.8.0 libQt5EglFSDeviceIntegration.so.5.8
Makefile.xcb_qpa_lib:4005: recipe for target '.obj/qxcbkeyboard.o' failed
make[7]: *** [.obj/qxcbkeyboard.o] Error 1
make[7]: *** Waiting for unfinished jobs....
rm -f ../../../../lib/libQt5EglFSDeviceIntegration.so.5.8.0
mv -f libQt5EglFSDeviceIntegration.so.5.8.0 ../../../../lib/
rm -f ../../../../lib/libQt5EglFSDeviceIntegration.so
rm -f ../../../../lib/libQt5EglFSDeviceIntegration.so.5
rm -f ../../../../lib/libQt5EglFSDeviceIntegration.so.5.8
mv -f libQt5EglFSDeviceIntegration.so ../../../../lib/
mv -f libQt5EglFSDeviceIntegration.so.5 ../../../../lib/
mv -f libQt5EglFSDeviceIntegration.so.5.8 ../../../../lib/
make[7]: Leaving directory '/home/longchair/LibreELEC.tv/build.PlexMediaPlayer-Generic.x86_64-8.0-devel/qt5-5.8.0-beta/qtbase/src/plugins/platforms/eglfs'
( test e Makefile.eglfs-plugin || /home/longchair/LibreELEC.tv/build.PlexMediaPlayer-Generic.x86_64-8.0-devel/qt5-5.8.0-beta/qtbase/bin/qmake -o Makefile.eglfs-plugin /home/longchair/LibreELEC.tv/build.PlexMediaPlayer-Generic.x86_64-8.0-devel/qt5-5.8.0-beta/qtbase/src/plugins/platforms/eglfs/eglfs-plugin.pro -qtconf /home/longchair/LibreELEC.tv/build.PlexMediaPlayer-Generic.x86_64-8.0-devel/qt5-5.8.0-beta/qtbase/bin/qt.conf – -sysroot /home/longchair/LibreELEC.tv/build.PlexMediaPlayer-Generic.x86_64-8.0-devel/toolchain/x86_64-libreelec-linux-gnu/sysroot -prefix /usr/local/qt5 -hostprefix /home/longchair/LibreELEC.tv/build.PlexMediaPlayer-Generic.x86_64-8.0-devel -device-option CROSS_COMPILE=/home/longchair/LibreELEC.tv/build.PlexMediaPlayer-Generic.x86_64-8.0-devel/toolchain/bin/x86_64-libreelec-linux-gnu -device linux-Generic-g++ -release -v -opensource -confirm-license -shared -make libs -no-pch -no-icu -qt-xkbcommon -no-sql-sqlite2 -nomake examples -nomake tests -no-libjpeg -qpa xcb -opengl desktop -no-kms -no-directfb -qt-xcb -no-accessibility -skip qtactiveqt -skip qtandroidextras -skip qtdoc -skip qtvirtualkeyboard -skip qtcharts -skip qtdatavis3d -skip qtgraphicaleffects -skip qtlocation -skip qtmacextras -skip qtscript -skip qtsensors -skip qtserialport -skip qtcanvas3d -skip qtwayland -skip qtwinextras -skip qtserialbus -skip qtxmlpatterns -skip qttranslations -skip qtmultimedia -skip qtscxml -skip qtquickcontrols2 -skip qt3d -skip qtpurchasing -skip qtgamepad -skip qtwebview --force-debug-info ) && make -f Makefile.eglfs-plugin
In file included from qxcbconnection.cpp:45:0:
qxcbkeyboard.h:45:29: fatal error: xcb/xcb_keysyms.h: No such file or directory
compilation terminated.
Makefile.xcb_qpa_lib:3135: recipe for target '.obj/qxcbconnection.o' failed
make[7]: *** [.obj/qxcbconnection.o] Error 1

It looks like it doesn't find the xcb headers where they are of course in the qt tree.

Here is the config options we use for building it :

-sysroot
/home/longchair/LibreELEC.tv/build.PlexMediaPlayer-Generic.x86_64-8.0-devel/toolchain/x86_64-libreelec-linux-gnu/sysroot
-prefix
/usr/local/qt5
-hostprefix
/home/longchair/LibreELEC.tv/build.PlexMediaPlayer-Generic.x86_64-8.0-devel
-device-option
CROSS_COMPILE=/home/longchair/LibreELEC.tv/build.PlexMediaPlayer-Generic.x86_64-8.0-devel/toolchain/bin/x86_64-libreelec-linux-gnu-
-device
linux-Generic-g++
-release
-opensource
-confirm-license
-shared
-make
libs
-no-pch
-no-icu
-qt-xkbcommon
-no-sql-sqlite2
-nomake
examples
-nomake
tests
-no-libjpeg
-qpa
xcb
-opengl
desktop
-no-kms
-no-directfb
-qt-xcb
-no-accessibility
-skip
qtactiveqt
-skip
qtandroidextras
-skip
qtdoc
-skip
qtvirtualkeyboard
-skip
qtcharts
-skip
qtdatavis3d
-skip
qtgraphicaleffects
-skip
qtlocation
-skip
qtmacextras
-skip
qtscript
-skip
qtsensors
-skip
qtserialport
-skip
qtcanvas3d
-skip
qtwayland
-skip
qtwinextras
-skip
qtserialbus
-skip
qtxmlpatterns
-skip
qttranslations
-skip
qtmultimedia
-skip
qtscxml
-skip
qtquickcontrols2
-skip
qt3d
-skip
qtpurchasing
-skip
qtgamepad
-skip
qtwebview
--force-debug-info

We also use gcc 5.4.
The qmake.conf file that is used for the build is very simple as follows (copy of the qt nuc one :

#

1. qmake configuration for the Intel NUC DE3815TYKE targeting EGL/GLES either via KMS (eglfs) or Wayland
   #
2. Verified with a core-image-weston image produced by Yocto.

include(../common/linux_device_pre.conf)

NUC_CFLAGS = "-m64 -fPIC"
QMAKE_CFLAGS += $$NUC_CFLAGS
QMAKE_CXXFLAGS += $$NUC_CFLAGS
QMAKE_LFLAGS += $$NUC_CFLAGS

include(../common/linux_device_post.conf)

load(qt_config)

I'm fully available if you need any further information.