Build CTA with XRootD v5
The time has come to build CTA against XRootD 5. This implies few or no code changes but will involve some work with dependencies, linking and packaging.
We agreed that we will continue to version lock the XRootD version and obtain the XRootD packages from our local cache repo (as is the case with XRootD 4).
Packages for external consumption should be able to link against the standard XRootD 5 packages, e.g. from epel.