libgm2: Provide fallback GetTimeRealtime when clock_getttime is not available. This copies the approach used in libstdc++ if getttimeofday is available and clock_gettime is not, then use the former. libgm2/ChangeLog: * libm2iso/wrapclock.cc (GetTimeRealtime): Provide a fallback when clock_gettime is not available. Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>