Update CMakeLists.txt
Update CMakeLists.txt

fix compilation on older cmakes
jkhoogland/cereal,fawkes74/cereal,adasworks/cereal,adasworks/cereal,USCiLab/cereal,fawkes74/cereal,USCiLab/cereal,jkhoogland/cereal,abutcher-gh/cereal,abutcher-gh/cereal