portconfigure.tcl: use clang5+ for cxx11+ if not libc++
portconfigure.tcl: use clang5+ for cxx11+ if not libc++

only macports-clang-5+ clang compilers will accept
macports-libstdc++

closes: https://trac.macports.org/ticket/59717

neverpanic/macports-base,macports/macports-base,danchr/macports-base,danchr/macports-base,macports/macports-base,neverpanic/macports-base