remove undef on CEREAL_STATIC_CONSTEXPR
remove undef on CEREAL_STATIC_CONSTEXPR

USCiLab/cereal,abutcher-gh/cereal,fawkes74/cereal,fawkes74/cereal,USCiLab/cereal,adasworks/cereal,abutcher-gh/cereal,jkhoogland/cereal,adasworks/cereal,jkhoogland/cereal