[cmake] no cxx extensions (#6664)
[cmake] no cxx extensions (#6664)

This commit sets cxx stand as -std=c++11 instead of -std=gnu++11.
abtink/openthread,abtink/openthread,abtink/openthread,srickardti/openthread,jwhui/openthread,srickardti/openthread,srickardti/openthread,openthread/openthread,jwhui/openthread,openthread/openthread,abtink/openthread,srickardti/openthread,jwhui/openthread,openthread/openthread,jwhui/openthread,openthread/openthread