Build with Meson (master)
This MR shows how gtkmm-4 can be built with Meson instead of Autotools. It's similar to the Meson build system in gtkmm-3.
There are some differences between gtkmm-4 and gtkmm-3.
- Building gtkmm-3 creates two library files (gdkmm-3.0 and gtkmm-3.0). Building gtkmm-4 creates only one public library file (gtkmm-4.0).
- Gtkmm-4 has less conditionally used source code than gtkmm-3. (some not used on Windows, but none used only on X11, and none deprecated)