Build with Meson (gtkmm-3-24)
This MR shows how gtkmm-3 can be built with Meson instead of Autotools. It's similar to the Meson build system in atkmm-1.6 and glibmm-2.4.
There are some differences between gtkmm-3 and other *mm modules that have got Meson support.
- Building gtkmm-3 creates two library files (gdkmm and gtkmm). This is similar to glibmm (glibmm and giomm).
- Gtkmm depends on modules (atkmm and pangomm) that use gmmproc. This means that atkmm's and pangomm's installed M4 files must be found.
- Gtkmm-3 has more conditionally used source code than other modules. (some deprecated, some not used on Windows, some used only on X11)