-rdynamic is necessary for clang as well to properly link shared
executables. Add furthermore an option to ignore unused command line
arguments, to suppress warning

  clang: warning: argument unused during compilation: '-rdynamic' [-Wunused-command-line-argument]

This is likely because -rdynamic is only relevant for the linker, but
the corresponding option add_link_options() is only available starting
cmake 3.13, while we are still at 3.12 (for Ubuntu 18, to my
understanding).