Created attachment 3064 
missing libxfcegui4 cflags/libs

because xfce4-panel-4.7 is using libxfce4ui instead of libxfcegui4, this needs to be explicitly defined or the build bails out on:

xfce4_genmon_plugin-main.o: In function `genmon_create_options':
main.c:(.text+0x15dd): undefined reference to `xfce_create_header'
xfce4_genmon_plugin-main.o: In function `About':
main.c:(.text+0x1d2e): undefined reference to `xfce_info'
collect2: ld returned 1 exit status

full build.log is here: 

http://bugs.gentoo.org/attachment.cgi?id=241751&action=view

attached patch solves the issue