Add libc++ package (libcxx)
Set proper compilers
Add paths to libsupc++

These paths are the first two paths from:

    $ echo | g++ -Wp,-v -x c++ - -fsyntax-only
    ignoring nonexistent directory "/usr/local/include/x86_64-linux-gnu"
    ignoring nonexistent directory "/usr/lib/gcc/x86_64-linux-gnu/4.6/../../../../x86_64-linux-gnu/include"
    #include "..." search starts here:
    #include <...> search starts here:
     /usr/include/c++/4.6
     /usr/include/c++/4.6/x86_64-linux-gnu/.
     /usr/include/c++/4.6/backward
     /usr/lib/gcc/x86_64-linux-gnu/4.6/include
     /usr/local/include
     /usr/lib/gcc/x86_64-linux-gnu/4.6/include-fixed
     /usr/include/x86_64-linux-gnu
     /usr/include
    End of search list.

Unfortunately this is platform dependent, so we need to possibly let the user
set these paths in default.yaml.
Add libcxxabi
Use CPLUS_INCLUDE_PATH
Add the path to libc++
Fix the rpath
Move the platform specific paths to host-libstdcxx
Add an example for Clang into examples/
Merge pull request #271 from hashdist/libcxx

Add libc++ for Clang