Enabling both EGLFS on the RPI as well as X11 produces an error while compiling, the file; '$Qt-everywhere-souce/qtbase/src/plugins/platforms/xcb/gl_integrations/xcb_egl/qxcbeglwindow.cpp', produces an error and compilation fails.

The error states:
'invalid conversion from 'xcb_window_t {aka unsigned int}' to 'EGLNativeWindowType {aka void*}' [-fpermissive]'.

changed the line (96);
_m_surface = eglCreateWindowSurface(m_glIntegration->eglDisplay(), m_config, m_window, 0);_

to:
_m_surface = eglCreateWindowSurface(m_glIntegration->eglDisplay(), m_config, (void*)m_window, 0);_

Resolved the issue, though this might break some compatibility checks, or it at least creates a working install for the raspberry pi.

The configure script I'm using;

_PKG_CONFIG_LIBDIR=/usr/lib/arm-linux-gnueabihf/pkgconfig:/usr/share/pkgconfig _
_PKG_CONFIG_SYSROOT_DIR=/ _
_../src-5.12.3/configure _
_-v _
_-make libs -prefix /usr/local/qt5 _
_-opensource -confirm-license -optimized-qmake _
_-reduce-exports -release -no-pch -qt-pcre -force-pkg-config _
_-opengl es2 -eglfs -tslib _
_-xcb -xcb-xlib -linuxfb _
_-device linux-rasp-pi-g++ _
_-device-option CROSS_COMPILE=/usr/bin/ _
_-skip qtlocation _
_-skip qtwayland _
_-skip qtwebengine _
_-skip qtscript _
_-ssl -evdev _
_-fontconfig _
_-glib _
-qpa xcb

I've also adjusted the 'linux-rasp-pi-g++' qmake.conf with the CFLAGS from 'linux-rasp-pi3-vc4-g'. I've attached the output log from the 'configure'.