Use @basename@ instead of @filename@ for reproducible builds
Whilst working on the Reproducible Builds effort [0], we noticed [1] that gdk-pixbuf could not be built reproducibly as it includes the absolute build path.
The glib-mkenums documentation recommends using @basename@ over @filename@ for this reason [2].
[0] https://reproducible-builds.org/ [1] https://bugs.debian.org/908309 [1] https://developer.gnome.org/gobject/stable/glib-mkenums.html#id-1.5.2.5.4