Fix _LIBCPP_VERSION with stdlib=libc++

problem with libboost_*

The problem initially reported is most likely caused by incompatibilities between `libstdc++` and `libc++`. Possible ways to fix that are to fully reinstall the packages and to make sure to always use clang to build the packages.

> ABI compatibility with gcc's libstdc++ for some low-level features such as exception objects, rtti and memory allocation.

http://libcxx.llvm.org/

or create patch [`hhvm/CMake/HPHPCompiler.cmake`](https://github.com/facebook/hhvm/blob/master/CMake/HPHPCompiler.cmake#L50-L54)

Found information this:
- http://stackoverflow.com/a/19774902/2126990
- http://stackoverflow.com/questions/10116724/clang-os-x-lion-cannot-find-cstdint
- http://stackoverflow.com/questions/13219007/xcode-4-5-tr1-type-traits-file-not-found
- http://stackoverflow.com/questions/22531247/detect-whether-clang-is-compiling-using-c11-or-legacy-libstdc-standard-libra
- http://stackoverflow.com/questions/8454329/why-cant-clang-with-libc-in-c0x-mode-link-this-boostprogram-options-examp