1.) If building with external iconv, remove remnants of libiconv data
created if previousy used internal iconv
2.) Remove the variable HELP from cache to enable subsequent builds.
(Different variable-handling on new cmake (version >= 3.18))
3.) New macro to extract cmake-known-cxx-sts features
4.) Check for CXX11-features only for a subset of possibilities
known by the used cmake
2ec243d47b