No fonts in Emacs (on Ubuntu) ?

A few days ago I downloaded and installed Emacs 28.1, and then I compiled it on my fresh Kubuntu Neon system.
When I launched my fresh Emacs, I had a surprise: the font system was not correctly initialized and the only default font I could choose from was between courier, misc and family.
I started working with the system font cache fc-cache and related commands in the hope to fix the problem. But nothing worked.
The packaged Emacs was working fine, but it was too old since Kubuntu installed Emacs 26!

After a few days, the solution came from the emacs mailing list: I was missing the GTK3 development libraries and therefore when compiling Emacs it was failing back to the default fontset.
The solution therefore was as simple as sudo apt install libgtk-3-dev and issue an Emacs compile cycle.