Several new stable versions of libxml++ have been released

[Kjell Ahlstedt <kjellahlstedt gmail com>]

libxml++ 4.0.0 and libxml++ 5.0.0 are the first stable releases of the libxml++-4.0 and libxml++-5.0 ABIs.
libxml++ 2.42.0 and libxml++ 3.2.2 were released a few months ago.

These 4 releases can all be built with Meson.

For a brief explanation of the differences between the 4 ABIs, see https://libxmlplusplus.github.io/libxmlplusplus/