Log Message:
libsigc++3: update to 3.2.0.

Remove some unused build dependencies.

3.2.0 (stable)

* Allow slots with rvalue reference parameters
  (Slava Andrejev) Pull requests #74, #77

Build:
* Meson build: Perl is not required by new versions of mm-common
  (Kjell Ahlstedt)
* NMake Makefiles: Support building with VS2022
  (Chun-wei Fan)

Documentation:
* Upgrade the manual from DocBook 4.1 to DocBook 5.0
  (Kjell Ahlstedt)
* ptr_fun(), mem_fun() docs: Remove left-overs from sigc++-2.0
  (Kjell Ahlstedt)
* Fix links to sigc::slot and sigc::signal
  (Kjell Ahlstedt)