Log Message:
libsigc++3: updated to 3.0.6

3.0.6 (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

3.0.4 (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 3.0.4 has been created with 'meson dist'.
If you build with Autotools from the tarball, please read the relevant
part of README.md.