CMake: forward CMAKE_CXX_STANDARD to depedent packages
Created by: andresailer
BEGINRELEASENOTES
- Pass CMAKE_CXX_STANDARD to externalproject_add to allow building packages with standards other than the compiler default or c++11
ENDRELEASENOTES
C++11 is still the default, because USE_CXX11 is true by default
We need a fix so that the nightly builds are running through again, as dd4hep now requires 14