From https://github.com/xbmc/xbmc/tree/Matrix I downloaded Kodi Matrix.

Via slackbuilds.org I installed
gtest
zuluopenjdk
libass
tinyxml
rapidjson
platform
(pycryptodomex and libcec for CEC)

Internal builds
make -C tools/depends/target/crossguid PREFIX=/usr/local
make -C tools/depends/target/flatbuffers PREFIX=/usr/local
make -C tools/depends/target/libfmt PREFIX=/usr/local
make -C tools/depends/target/libspdlog PREFIX=/usr/local

configuration by
cmake .. -DCMAKE_INSTALL_PREFIX=/usr/local -DCORE_PLATFORM_NAME=gbm -DAPP_RENDER_SYSTEM=gles -DCMAKE_INSTALL_PREFIX=/usr/local -DENABLE_INTERNAL_CROSSGUID=ON -DENABLE_INTERNAL_FSTRCMP=ON

compiling successfully

running in the tree by ./kodi-gbm gives
EROR: Unable to create GUI. Exiting

What does it mean and how to get kodi running?

You are missing dav1d and libnfs.
Please check out my slackbuild script for kodi 19.5 and 20.1.
Also have some prebuild slackware 15.0 arm kodi 19.5 and kodi 20.1 packages built for both x11 and gbm.

On an x86 processor you might get this error "EROR: Unable to create GUI. Exiting" if you have upgraded xorg-server and not reisntalled the graphics driver.
Note you cannot run gbm on an x86 computer as it does not support gbm, you will have to run kodi from x11.

These are not required, see slackbuilds.org

I used your build script to build the x11 version. It builds like a charm. Executing the binary kodi-standalone results in:

ERROR: Unable to create GUI. Exiting
ERROR: Unable to create GUI. Exiting
ERROR: Unable to create GUI. Exiting
/usr/bin/kodi --standalone has exited in an unclean state 3 times in the last 5 seconds.
Something is probably wrong

I wonder why.

I installed your kodi-x11 Matrix binary yesterday: yesterday it ran nicely. Today I installed the one using your buildscript and removed it. Reinstalled your binary running now 320% cpu time?? The kodi-gbm gives the "Unable to Create GUI" error.

You are right.

Remark cpu load of 320% disappeared. I cannot control Kodi by remote control of television despite of /dev/cecX exists.

I configured Kodi for X11 by

cmake .. -DCMAKE_INSTALL_PREFIX=/usr/local -DCORE_PLATFORM_NAME=x11 -DAPP_RENDER_SYSTEM=gl -DCMAKE_INSTALL_PREFIX=/usr/local -DENABLE_INTERNAL_CROSSGUID=ON
-DENABLE_INTERNAL_FSTRCMP=ON

I could compile it for 80% and stopped finally due to the open GL error:

error: ‘glFinish’ was not declared in this scope

What arch are you compiling on? rpi does not support "gl", you have to use "gles".

configuration by
cmake .. -DCMAKE_INSTALL_PREFIX=/usr/local -DCORE_PLATFORM_NAME=x11 -DAPP_RENDER_SYSTEM=gles -DCMAKE_INSTALL_PREFIX=/usr/local -DENABLE_INTERNAL_CROSSGUID=ON -DENABLE_INTERNAL_FSTRCMP=ON

resulted in a good build.

HDMI-CEC is not supported: I cannot use the remote control of the television to control Kodi. This behaviour was also in the binary of Minime_2003. Under Raspberry Pi OS Bullseye (and Buster) it is supported.

Thanks Minime_2003 to attent me on to use X11 with GLES.