Log Message:
Update libsigc++ to 2.10.0

Given "Some more minor uses of C++11 syntax." below, up GCC requirement
to 4.8 and add -std=c++11 to be consistent with e.g., glibmm.

2.10.0 (stable):

* Build: Fix silent builds.
  (Kjell Ahlstedt) Bug #768797

2.9.3 (unstable):

* signal:
  Deprecate slots().
  Please tell us if you really need this.
  (Murray Cumming)
  Deprecate emit_reverse().
  Please tell us if you really need this.
  (Murray Cumming)
* Benchmark: Update it and use boost::timer, and actually build it,
  but not built by default.
  (Murray Cumming)

Build:
* Fix the build on MSVC++ 2013 and 2015.
  (Chun-wei Fan) Bug #767777

2.9.2 (unstable):

* Minor documentation improvements.
  (Murray Cumming)
* Some more minor uses of C++11 syntax.
  (Murray Cumming)

2.9.1 (unstable):

* slot: Allow sigc::slot<R(Args...)> syntax, like std::function,
  deprecating the sigc::slot<R, Args...> syntax.
  (Murray Cumming, Kjell Ahlstedt) Bug #763393
* signal: Allow sigc::signal<R(Args...)> syntax, like std::function,
  deprecating the sigc::signal<R, Args...> syntax.
  (Murray Cumming, Kjell Ahlstedt) Bug #763393