GTK 3 meson support

[reiter christoph gmail com]

Hey everyone,

the latest GTK 3 release (3.24.9) contains experimental meson build support.

It's not 100% complete yet as it's lacking installed tests and the mir backend
and there is ongoing work to improve various details, but things are looking
good so far.

Related issues/MRs:

Issues: 
https://gitlab.gnome.org/GNOME/gtk/issues?scope=all&utf8=%E2%9C%93&state=all&label_name[]=Build&label_name[]=GTK3
Merge Requests: 
https://gitlab.gnome.org/GNOME/gtk/merge_requests?scope=all&utf8=%E2%9C%93&state=all&label_name[]=Build&label_name[]=GTK3

If you find a problem or have any suggestions for improvements please file an
issue.

Note that there are currently no plans to the remove the autotools based build
system.

regards