Since I wrote a blog post about building GTK4 applications on Windows, someone asked about gtkmm, so I gave it a try.
https://www.collabora.com/news-and-blog/blog/2021/04/29/build-your-own-application-with-gtk4-as-a-meson-subproject/

I noticed a few things that can be improved:

• Fallback for find_program('mm-common-get', required: false) does not work. If the program is not required Meson prefer not using the fallback subproject. Changing it to find_program('mm-common-get', required: maintainer_mode) would make more sense since you error() just below anyway.
• Building doc requires many extra deps (perl, etc), would be nice to make that optinal by default using a feature option that defaults to auto.
• When disabling documentation, 'doxytagfile': tag_file.full_path() fails because tag_file is not defined.