This also changes emper_log so that a std::ostringstream is used to
assemble the log message.

Co-authored-by: Florian Fischer <florian.fl.fischer@fau.de>

And fix the #include issues reported by the newer IWYU version.

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.