VS2019 / C++ 17, signal::disconnect compile error: syntax error: 'template'

Using custom functor, Signals fail to compile with:
signal_template.hpp(537,50): error C2059: syntax error: 'template'

Here:
```
// check for wrapped extended slot
bound_extended_slot_function_type *fp;
fp = (*it)->slot().slot_function().template target();
```

The reason behind is that the signal expects the functor to be the boost::function / std::function, that would contain the target() method.

Minimal example:
```
struct MyFunctorThisComparer
{
MyFunctorThisComparer(const void* this_) {}
bool operator==(const MyFunctorThisComparer& other) const { return true; }
};

struct MyFunctor
{
typedef void result_type;

template
void operator=(F) {}
void operator()() {}

// disconnect by comparison with custom predicate
bool operator==(const MyFunctorThisComparer& other) const { return true; }
operator MyFunctorThisComparer() const { return MyFunctorThisComparer { nullptr }; }
};

void signals_disconnect()
{
boost::signals2::signal<
void(),
//SignalSupport_::AllCall::result_type>,
boost::signals2::optional_last_value::result_type>,
int,
std::less,
MyFunctor > s;

s.connect( &signals_disconnect );
s.disconnect( MyFunctorThisComparer(nullptr) );
}
```

I attempted here to provide the equivalence MyFunctorThisComparer predicate, that would filter out functors. In particular, it would know how to extract 'this' pointer from MyFunctor. However, it could be a more generic predicate.

Boost 1.72.0. However, I have checked the development version - it has the same lines there, so the issue is still present.