libsigc++: updated to 2.10.6

2.10.6 (stable)

Build:
* Meson build: Fix versioning on macOS

Documentation:
* sigc++/sigc++.h: Describe how to use libsigc++ with Meson
* Update links to the web page, now at
  libsigcplusplus.github.io/libsigcplusplus

2.10.4 (stable)

Build:
* Meson build: Set default value of the 'warnings' option to 'min'
* NMake Makefiles: Several improvements
* docs/reference/: Update for Doxygen >= 1.8.16

Documentation:
* docs/manual/libsigc_manual.xml: Update signal and slot syntax

The tarball for 2.10.4 has been created with 'meson dist'.
If you build with Autotools from the tarball, please read the relevant
part of README.

(adam)