Log Message:
libsigc++: update to 2.10.3.

2.10.3 (stable)

Build:
* Add Meson build, alongside the existing Autotools build.
  (Kjell Ahlstedt, Chun-Wei Fan) Pull request #47, #50
* Improve MSVC builds on Windows
  (Chun-Wei Fan) Pull request #50
* meson.build: Check if .git is a directory or file
  (Kjell Ahlstedt) Merge request pangomm!8 (Ting-Wei Lan)
* docs/reference/meson.build: Check if perl is found
  (Kjell Ahlstedt) Issue #53 (Rasmus Thomsen)
* README: Describe building with Meson and Autotools
  (Kjell Ahlstedt)

Tests:
* Make test_track_obj.cc compile with clang++
  (Kjell Ahlstedt)