http://bugs.gentoo.org/show_bug.cgi?id=289445#c2

--- src/x86_64/unwind_i.h
+++ src/x86_64/unwind_i.h
@@ -65,6 +65,12 @@
 extern void x86_64_local_addr_space_init (void);
 extern int x86_64_local_resume (unw_addr_space_t as, unw_cursor_t *cursor,
 			     void *arg);
+
+/* implemented in setcontext.S */
+#if defined(__linux)
+extern int _Ux86_64_setcontext (const ucontext_t *ucp);
+#endif
+
 #if 0
 extern dwarf_loc_t x86_64_scratch_loc (struct cursor *c, unw_regnum_t reg);
 #endif