When compiling netload plugin with xfce4-panel from git it writes this:

gcc -DPACKAGE_LOCALE_DIR=\"/usr/local/share/locale\" -pthread -I/usr/include/xfce4/libxfce4panel-1.0 -I/usr/include/gtk-2.0 -I/usr/include/glib-2.0 -I/usr/lib64/glib-2.0/include -I/usr/include/xfce4 -I/usr/lib64/gtk-2.0/include -I/usr/include/atk-1.0 -I/usr/include/cairo -I/usr/include/pango-1.0 -I/usr/include/pixman-1 -I/usr/include/freetype2 -I/usr/include/libpng14   -g -O2 -Wl,--export-dynamic -pthread -lxfce4panel-1.0 -lgtk-x11-2.0 -lxfce4util -lgdk-x11-2.0 -latk-1.0 -lgio-2.0 -lpangoft2-1.0 -lgdk_pixbuf-2.0 -lpangocairo-1.0 -lcairo -lpango-1.0 -lfreetype -lfontconfig -lgobject-2.0 -lgmodule-2.0 -lgthread-2.0 -lrt -lglib-2.0    -o xfce4-netload-plugin xfce4_netload_plugin-netload.o xfce4_netload_plugin-utils.o xfce4_netload_plugin-net.o  
xfce4_netload_plugin-netload.o: In function `setup_monitor':
/home/plisk/tmp/xfce4-netload-plugin/panel-plugin/netload.c:468: undefined reference to `xfce_err'
xfce4_netload_plugin-netload.o: In function `monitor_create_options':
/home/plisk/tmp/xfce4-netload-plugin/panel-plugin/netload.c:849: undefined reference to `xfce_titled_dialog_new_with_buttons'
collect2: ld returned 1 exit status
make[2]: *** [xfce4-netload-plugin] Error 1

A bit of googling suggested to add libxfcegui4 to linking flags and with it compiles and works okay. Attaching trivial patch with this.