[cmake] Quote cmake variable expansion
[cmake] Quote cmake variable expansion

Otherwise we get cmake errors when CLING_CXXFLAGS is empty.

olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root