make the userspace-rcu dependency optional
Disable the userpace-rcu support by default. Our used userspace-rcu flavor memb is rather new and not available in liburcu version 0.10 available in debian buster. This change switches from using DCE and "if constexpr" to the C preprocessor so the library is only needed when the userspace-rcu support is actually enabled.
Motivation is building the emper on our bigboxes.
Edited by Maxim Onciul