[boost] Boost.Config for clang on Linux with libc++

Hi, Attached are user.hpp and the console output from running configure with clang-3.9. Note that I tried to configure using libc++ but the configure script immediately failed, so these are the results for clang-3.9 with the default stdlib (libstdc++). Can someone please take a look and see what adjustments might need to be made to Boost.Config to support this configuration? Once it works for the default stdlib, I’d like to rerun it to use libc++. Thanks, Noel Belcourt