Fix build without libsml

vzlogger can now be build without libsml.
there were just two ifdefs in Meter.cpp missing,
and cmake should not abort if libsml was not found.
(config to conditonally compile MeterSML.cpp was already present)

(this could be cleaned up a little, we could abort if sml was explicitly requested but not found, but i'm not that familiar with cmake either)