Steps to reproduce:
The problem is that there's a class also named 'internal' in type_traits.h.  
Renaming this class solves the problem.

Stack trace:
Trying to build you will get:

..\sigc++\functors\slot_base.h(29) : error C2757: 'internal' : a symbol with 
this name already exists and therefore this name cannot be used as a namespace 
name

and similar errors for other sources.

The error is in file type_traits.h.  In a nutshell:

namespace sigc {
    struct is_base_and_derived {
        friend struct internal;
    };
}

namespace sigc {
    namespace internal {
    }
}

Comeau online doesn't like the code either.

Other information: