I cannot build from source because I am unable to satisfy the dependency of libfm-gtk3.
Old Ubuntu versions had libfm-gtk1, and recent versions have libfm-gtk4.
I reckon that for Ubuntu environments, the ./configure script will have to be adapted a little more.
The dependency package you seek would be named libfm-gtk3-4, not libfm-gtk3. But unfortunately for you, lxpanel currently isn't buildable against GTK+ 3.0. I believe it will be later so just use GTK+ 2.0 version now, GTK+ 2.0 is not worse than GTK+ 3.0. Thank you very much.
It should be buildable basically with current GIT sources, it even works but task buttons don't show any text or icons from other desktops, I have no idea why.