This package uses Vala in such a way that the generated C code is not C99-compatible. C99 compatibility of internal setters <https://discourse.gnome.org/t/c99-compatibility-of-internal-setters/13360> valac does not respect internal header/vapi setting <https://gitlab.gnome.org/GNOME/vala/-/issues/358> We need to build in C89 mode instead.
Thanks for the fix Florian!