is there a way to install self compiled panel plugins in the user's home dir?
If you configure a plugin (e.g. "genmon" in my case) with "configure --prefix=$HOME/software", then compile and install it, it is actually installed in $HOME/software/libexec/xfce4/panel-plugins/xfce4-gemon-plugin, but the xfce panel does not recognize the new file. :?
Yes this is possible, xfce4-panel looks in the home directory, and IIRC it look in the $XDG_DATA_HOME directory. So when this directory is (for example) ~/.local, the plugin will be installed in ~/.local/libexec/xfce4/panel-plugins/, with the desktop file in ~/.local/share/xfce4/panel-plugins.
So echo $XDG_DATA_HOME and see if it exists, then you can compile with configure --prefix=$XDG_DATA_HOME && make && make install
Thanks for the hint. It works now! ;D