Merge branch 'wip/baedert/test-cflags' into 'master'

Pass (almost) all our cflags to tests, testsuite, examples, demos, ...

See merge request GNOME/gtk!1516

diff --cc demos/gtk-demo/application.c
Simple merge

diff --cc gtk/gtkfilechooserwidget.c
Simple merge

diff --cc tests/testgtk.c
Simple merge

diff --cc testsuite/gtk/meson.build
Simple merge