3.0.1 (stable)

* Build:
  - Add meson build, alongside the existing autotools and CMake builds.
  (Kjell Ahlstedt, Chun-Wei Fan)
  - CMake: Fix the MSVC build, enable warnings, and disable unnecessary
  warnings, with MVSC.
  (Stuart Dootson)
* connection: Allow copying of empty connections.
  (Aigrind)
* sigc++/adaptors/bind.h: Make bind_functor::bound_ public.
  (Used by sigc::visitor::do_visit_each().
  (Kjell Ahlstedt) Bug #26
* test_limit_reference: Avoid virtual-move-assign warning
  (seen with g++ 9.2)
  (Murray Cumming)
* signal: slot_iterator_buf: Remove unused T_result default type
  (Murray Cumming)
* tests: Slight cleanups
  (Murray Cumming)
* Remove unused #ifdefed code.
  (Murray Cumming)