libsigc++: updated to 2.10.7

2.10.7 (stable)

Meson build:
* Make it possible to use sigc++ as a subproject
* Fix dependency on files generated from .h.m4 files
* No implicit_include_directories
* Make quiet installations possible
* Fix build as subproject without building documentation

Documentation:
* Links to *.gnome.org refer to sigc++-2.0 and gtkmm-3.0 versions
  instead of latest versions

(adam)