Add missing call to _g_io_modules_ensure_extension_points_registered() to
GRegistryBackend, GNullSettingsBackend, and GNextstepSettingsBackend
(cherry-picked from commit 924da751c2)
Those files got renamed to .c to work around an automake issue, but
Meson needs them to have .m extension. Better rename them at build time
in Makefile.am since that's where the workaround is needed.
https://bugzilla.gnome.org/show_bug.cgi?id=672777