Log Message:
devel/libsigc++3: import libsigc++3-3.0.0

libsigc++ implements a full callback system for use in widget
libraries, abstract interfaces, and general programming. Originally
part of the Gtk-- widget set, libsigc++ is now a separate library
to provide for more general use. It is the most complete library
of its kind with the ability to connect an abstract callback to a
class method, function, or function object. It contains adaptor
classes for connection of dissimilar callbacks and has an ease of
use unmatched by other C++ callback libraries.

This is major version 3 of the library.