I don't think the prototypes for exo_atomic_inc and exo_atomic_dec are needed and the break make distcheck here when building xfce4-panel.
Created attachment 1779 Remove static prototypes in header This fixes the problem. The functions are always defined (either an inline version or a define to the glib functions), so I don't think they are needed and only causing problems.
*** This bug has been marked as a duplicate of bug 4387 ***