[sigc] Move constructor for slots not selected

[Andrejs Hanins <andrejs hanins ubnt com>]

Hi,

        I noticed that both in 2.6.1 and current trunk (cabc88f) the move constructor for the slot is not 
selected by the compiler (gcc 4.9 and 4.8.4 tried) even in the most trivial case. Take test_slot_move.cc unit 
test for example:

sigc::slot<void,int> s2(std::move(s1));

If you now do (bool)s1 it will evaluate to 'true'. Also s1() works and calls foo() as if s1 wasn't moved to 
s2. I tried to add traces to slot1 copy and move constructors and indeed only copy constructor is called, but 
not move. So, something is wrong here - either compiler bug (unlikely) or some code in the 
slot/slot_base/functor_base prevents compiler from selecting move semantics for constructor.

BR, Andrey