This is a combined upgrade and overhaul of port:libcxx :
- upgrade to 6.0.0
- build using the preferred/official CMake buildsystem instead of
  through a restored and patched buildit script. One advantage of
  this way of building is that there is no stdlib.h bug and one
  can use recent port:clang versions (tested with Clang 5.0.1).
- adds a +runtime variant which allows (older (libc++-based)) systems
  to test and run applications with a current libc++/libc++abi version
  via DYLD_INSERT_LIBRARIES .

(This allows the use of LTO even with +universal)

that is, when a libc++ copy is already installed. Otherwise,
fall back to the bootstrap build using the buildit script.