Created attachment 3067 
Fix building with exo-0.5 and xfce4-panel-4.7

xfce4-mpc-plugin is working fine with exo-0.5, but the configure.ac check doesn't detect it:

checking for exo-0.3 >= 0.3.1.1... not found
*** The required package exo-0.3 was not found on your system.
*** Please install exo-0.3 (atleast version 0.3.1.1) or adjust

after which it will fail to compile with xfce4-panel-4.7 because it's using libxfce4ui instead of libxfcegui4, so need explicit linking now:

xfce4_mpc_plugin-xfce4-mpc-plugin.o: In function `mpc_show_about':
xfce4-mpc-plugin.c:(.text+0xf67): undefined reference to `xfce_about_info_new'
xfce4-mpc-plugin.c:(.text+0xf8e): undefined reference to `xfce_about_info_add_credit'
xfce4-mpc-plugin.c:(.text+0xf9b): undefined reference to `xfce_about_info_set_homepage'
xfce4-mpc-plugin.c:(.text+0xfaa): undefined reference to `xfce_themed_icon_load'
xfce4-mpc-plugin.c:(.text+0xfba): undefined reference to `xfce_about_dialog_new_with_values'
[ .. ]

The attached patch solves both issues at once.

And sorry again for reporting to xfce4-panel product, it's because "Panel Plugins" is missing "xfce4-mpc-plugin". Please reassing to correct maintainer, thanks!