Yeah, I tried to compile a minimum example now, which I try to compile with:
g++ -Wall -std=c++11 -I/Users/Shared/flens FlensMinimalExample.cxx
However, I still get the same compilation errors when compiling with clang 10.0.1, but it compiles when using gcc 7.4.0 from macports. Anyway, totally right that it’s off-topic, so I will have to switch over to stackoverflow here.
Thanks for all your help!