Applied changes suggested by @broulik.
Thank you for your help.

Reverted to the "old way" of trying ddcutil compatibility of the detected displays: by trying to read the feature setting. The ddca_get_feature_list_by_dref function was giving inconsistent results when used right after boot... (logging out an in again seemed to help, but not always..).

I went a bit back to it: it is again in a working state with the current DDCUtil API.
Still, single slider independently of the number of displays supporting ddc connected.
Tested with my dell 2212hm and a Magedok t116 via HDMI on AMD gpu,

Yes it has to be modified. I got it to compile again over lunch on a laptop, however could not test it (has to be on a desktop, since powerdevil picks integrated screens first for brightness).

In D8626#443079, @ngraham wrote:

Probably because @dvogel doesn't have commit access and nobody ever landed the patch on his behalf. :( Feel free to do so now I guess, since @kbroulik has accepted it.

Applied changes to the CMakeList.
Somehow the previous diff was messed-up: downloading it would provide a diff with the wrong folder structure...

Applied changes suggested by broulik.
CMake fixed, using the checkbox for WITH_DDCUTIL in Kdevelop now works.
I must appologise for the long time this is taking. The diff is based on 5.12.4, since 5.12.80 is not available on my system (Arch). I will try to rebase.

Reworked for simplification.

applied comments from d_ed
fixed brightness jumping to previous position after brightness change: see DDCutilBrightness::brightness()

Absence of ddcutil on the system is now handled:
the ddcbrightness object is still instantiated in powerdevilupowerbackend, however, this is a dummy object, returning isSupported()=FALSE, making powerdevilupowerbackend avoid using ddcutil.

The final conclusion was that I should figure out some cmake to allow building without ddccontrol. I kind of did it locally (tho it's a bit dirty).
The second big point is that powerdevil only supports one brightness controller at a time. The solution kbroulik suggested, is that powerdevil should be refactored in some sort of plugin architecture. However I personally do not have any idea at all how that should look, and most probably the time to do it.

Addition of a QTimer set by default to 1 sec to filter setBrightness calls: the actual DDC communication happens 1 sec after the last setBrightness() call.
This solves brightness flickering when scrolling quickly on the battery icon, and communication failure over DDC when waking the monitor from power-save mode (Dell U2212HM wakes up in less than 1 sec).

Applied changes suggested by reviewers.
The only issue remaining is brightness restoration when waking up the monitor after shutting it down: we try to set brightness before the monitor is actually ready. Setting then fails, and the monitor wakes up with the last value it was dimmed to before shut down.
TODO: Make ddcutillib optional, for now not having ddcutil installed will most likely result in a crash.

Ok, so there is apparently an issue when the screen gets dimmed.