community/gtkmm3: upgrade to 3.24.8

Changelog:

3.24.8 (stable):

Gtk:
* gtk/gtkmm/base.h: Fix GTK_VERSION_GT_MICRO
  (Ole Christian Eidheim) Issue #135

Documentation:
* Remove README.SUN and other obsolete files
  (Kjell Ahlstedt) Issue #140

Build with Meson:
* Detect if we build from a git subtree
  (William Roy) Merge request !72
* Fix the evaluation of is_git_build on Windows
  (Kjell Ahlstedt) Issue #131 (William Roy)
* Don't copy files with configure_file()
  (Kjell Ahlstedt)