Fix #32

The stack compiles with these changes. I am not a CMake expert, but I guess there could be a better solution to extract the ROOT CXX standard than matching the preprocessor commands in RConfigure.h. Running root-config --cflags yields

$ lb-run ROOT/v6.20.02 root-config --cflags
-pthread -std=c++17 -m64 -I/cvmfs/lhcb.cern.ch/lib/lcg/releases/LCG_97/ROOT/v6.20.02/x86_64-centos7-gcc9-opt/include

which looks like it could be used, but I have no Idea if it is straight forward to access this information from within CMake.