@dimstar Apologies for the late reply. I honestly do not know much about using Linux to build Pencil2D, however QT framework is the same everywhere, you need the QT creator which is a graphical IDE.
Once you install it with all it’s dependencies, open the Qt creator and you’ll be prompted to associate a C++ compiler with QT5, this will be asked automatically and most people recommend GCC compiler for linux distros.
You should be able to open the project file (.pro) that comes with the source however remember that to download the source code you have to get it via ZIP file from the github repository (you can find those links on the right sidebar on this site)
With QT creator you just have to press the green arrow button that says build and you’ll be able to run the latest incarnation of the source code. I do not know how to make binaries though, so maybe a wiser linux user or developer might be able to help you out.