atkmm: updated to 2.28.1

atkmm 2.28.1 (stable):

The tarball for 2.28.1 has been created with 'meson dist'.
If you build with Autotools from the tarball, please read the relevant
part of the README file.

Build:
* Improve NMake support
* Use __declspec(dllexport) when building with Visual Studio
* Add Meson support
* doc/reference/: Update for Doxygen >= 1.8.16

atkmm 2.28.0 (stable):

This release is identical to 2.24.3.

There is no 2.26.x release.

(adam)