Bug#558581: Better fix?

[Hilko Bengen]

The extra function definition in tasklist.h is protected using the
following ifdef:

,----
| #ifdef WNCK_I_KNOW_THIS_IS_NOT_UPSTREAM
| void wnck_tasklist_set_orientation(WnckTasklist *tasklist,
|                                    GtkOrientation orient);
| #endif
`----

Adding

CFLAGS += -DWNCK_I_KNOW_THIS_IS_NOT_UPSTREAM

instead of suppressing the warning / error also fixes the compile error.

Cheers,
-Hilko