Because xfce4-panel is using libxfce4ui instead of libxfcegui4 the flags won't get inherited anymore.

xfce4_wavelan_plugin-wavelan.o: In function `wavelan_configure':
wavelan.c:(.text+0xed9): undefined reference to `xfce_create_header'

--- configure.in
+++ configure.in
@@ -49,6 +49,7 @@
 
 dnl configure the panel plugin
 XDT_CHECK_PACKAGE([LIBXFCE4UTIL], [libxfce4util-1.0], [4.3.20])
+XDT_CHECK_PACKAGE([LIBXFCEGUI4], [libxfcegui4-1.0], [4.3.20])
 XDT_CHECK_PACKAGE([LIBXFCE4PANEL], [libxfce4panel-1.0], [4.3.20])
 XDT_CHECK_PACKAGE([GTK], [gtk+-2.0], [2.6.0])
 
--- panel-plugin/Makefile.am
+++ panel-plugin/Makefile.am
@@ -11,7 +11,8 @@
 #	libwavelan.la
 
 xfce4_wavelan_plugin_LDFLAGS =							\
-				@LIBXFCE4PANEL_LIBS@
+				@LIBXFCE4PANEL_LIBS@ \
+				@LIBXFCEGUI4_LIBS@
 
 xfce4_wavelan_plugin_SOURCES =							\
 	wavelan.c							\
@@ -21,7 +22,8 @@
 	wi_linux.c
 
 xfce4_wavelan_plugin_CFLAGS =							\
-	@LIBXFCE4PANEL_CFLAGS@
+	@LIBXFCE4PANEL_CFLAGS@ \
+	@LIBXFCEGUI4_CFLAGS@
 
 # .desktop file
 #