looks like the DBG macro is only expanded when configure is run with --enable-debug=full. With the 
macro expanded, build fails with

iconbox_plugin.c: In function `dialog_response':
iconbox_plugin.c:796: warning: int format, pointer arg (arg 5)
iconbox_plugin.c:796: warning: format argument is not a pointer (arg 6)
iconbox_plugin.c:796: warning: too many arguments for format
make[2]: *** [iconbox_plugin_la-iconbox_plugin.lo] Error 1
make[2]: Leaving directory `/home/erik/xfce4-panel-4.3.2/settings'
make[1]: *** [all-recursive] Error 1
make[1]: Leaving directory `/home/erik/xfce4-panel-4.3.2'
make: *** [all] Error 2