Log Message:
atkmm: updated to 2.28.4

atkmm 2.28.4 (stable):

Documentation:
* atkmm.h: Update link to ATK
* Add README.win32.md with Win32 build documentation
* Remove AUTHORS and add general information to README.md

Meson build:
* Detect if we build from a git subtree
* Don't copy files with configure_file()
* Fix the evaluation of is_git_build on Windows
* Don't fail if warning_level=everything