macports-clang versions < 5.0 don't understand -stdlib=macports-libstdc++

[kencu (Ken)]

currently base will try to pass -stdlib=macports-libstdc++ to clang 3.4 or 3.7 under certain circumstances of blacklisting and whitelisting when the compiler c or c++ standards are set to >= 2011.

in ​portconfigure:get_min_clang the logic to select a proper clang doesn't take into account the fact that macports-clang versions < 3.9 won't be able to understand -stdlib=macports-libstdc++.

we don't have macports-clang-3.9 any more, so practically for us macports-clang versions < 5.0 can only be used for compiler.cxx_standard >= 2011 or compiler.c_standard >= 2011 if configure.cxx_stdlib=libc++.