Created attachment 3053 
fix LDFLAGS vs. LDADD typo and add libxfcegui4

full build.log here:

https://bugs.gentoo.org/attachment.cgi?id=241749&action=view

the important part:

xfce4_mailwatch_plugin-mailwatch-plugin.o: In function `mailwatch_help_clicked_cb':
mailwatch-plugin.c:(.text+0x3ce): undefined reference to `xfce_exec'
mailwatch-plugin.c:(.text+0x460): undefined reference to `xfce_message_dialog'
xfce4_mailwatch_plugin-mailwatch-plugin.o: In function `mailwatch_button_release_cb':
mailwatch-plugin.c:(.text+0x57c): undefined reference to `xfce_exec'
xfce4_mailwatch_plugin-mailwatch-plugin.o: In function `mailwatch_new_messages_changed_cb':
mailwatch-plugin.c:(.text+0xf10): undefined reference to `xfce_exec'
xfce4_mailwatch_plugin-mailwatch-plugin.o: In function `_xpp_realize':

[ ... ]

because xfce4-panel-4.7 is linked to libxfce4ui instead of libxfcegui4 so it won't be bringed in anymore

the attached patch solves the issue