devel/libsigc++12: unbreak build with Clang 6 (C++14 by default)

bind_test.cc:24:17: error: no viable conversion from '__bind<SigC::Slot1<void, int>, int>' to 'Slot0<void>'

Slot0<void> s=bind(slot(&foo),1);
            ^ ~~~~~~~~~~~~~~~~~~

../sigc++/slot.h:207:7: note: candidate constructor not viable: no known conversion from '__bind<SigC::Slot1<void, int>, int>' to 'const SigC::Slot0<void> &' for 1st argument

Slot0(const Slot0& s)
^

../sigc++/slot.h:210:7: note: candidate constructor not viable: no known conversion from '__bind<SigC::Slot1<void, int>, int>' to 'SigC::SlotNode *' for 1st argument

Slot0(SlotNode* node)
^

../sigc++/slot.h:213:7: note: candidate constructor not viable: no known conversion from '__bind<SigC::Slot1<void, int>, int>' to 'SigC::Slot0<void>::Callback' (aka 'void (*)()') for 1st argument

Slot0(Callback callback)
^

Reported by: antoine (via bug 224669)