community/libsigc++: upgrade to 2.12.1
2.12.1 (stable)
Documentation:
* slot, signal: Describe the slot<void(int)> syntax more
(Kjell Ahlstedt) Issue #86 (Daniel Boles)
* connection: Improve the class documentation
(Kjell Ahlstedt) Issue #88 (Daniel Boles)
* Improve Visual Studio build documentation
(Chun-wei Fan) Pull request #94
* Remove AUTHORS and add general information to README.md
(Kjell Ahlstedt) Issue gtkmm#140
Build:
* Meson: Don't copy files with configure_file()
(Kjell Ahlstedt)
* Meson: Fix the evaluation of is_git_build on Windows
(Kjell Ahlstedt) Issue gtkmm#131 (William Roy)
* Visual Studio: Support static builds
Issue #90 (Swat-SomeBug)
(Chun-wei Fan) Pull request #92