Drop support for versions of CUPS < 2.0, to simplify the backend code and drop a lot of conditional blocks.
The http* family of functions was deprecated after CUPS 1.7. We can conditionally use it when built against a newer version of CUPS. The additional parameters are taken directly from the fallback values inside CUPS itself.
This will let us use GTK_PATH to load them uninstalled, which is useful for in-tree tests.
