Allow building without GDK-Pixbuf
When building without GDK-Pixbuf, this disables the legacy pixbuf-based
C API (rsvg_pixbuf_*
), along with these functions:
rsvg_handle_get_pixbuf()
rsvg_handle_get_pixbuf_and_error()
rsvg_handle_get_pixbuf_sub()
Context: https://github.com/lovell/sharp-libvips/issues/226.