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.

The default flavor is the preferred way to use the userspace library it uses
the sys_membarrier syscall or falls back to using userspace membarriers.

This adds the liburcu dependency which must be installed regardless the
'liburcu' meson option.

All options in meson_options.txt are exported in emper-config.h as
uppercase defines and prefixed with EMPER_.
Every source file using one of the EMPER_* defines now includes "emper-config.h".