Compile-time:
      * A new meson option 'log_level' is introduced which has all the values from the
        Loglevel enum
      * The 'OFF' option defines EMPER_LOG_OFF which causes the LOG* macros to be empty
        and the Logger<>::log function to return immediately
      * The default 'automatic' option sets 'log_level' to 'Error' for release builds
        and to 'ALL' otherwise

    Runtime:
      * The global variable emper::log_level defined in Debug.hpp controls the logging output
      * It is initialized with the meson option 'log_level' but can be changed during runtime

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.