Created attachment 2929 Compile-fine-with-libxfce4panel-4.7 attached is a patch to apply against git tree. with panel 4.7 one can only include libxfce4panel/libxfce4panel.h and libxfcegui4 is no more a dependency of it (which used to be pulled via pkg-config).
*** This bug has been marked as a duplicate of bug 6382 ***
in 4.8.0