Add libc++ for Clang

It seems to be working for me, use it like this to compile other projects using Clang + libc++:
```
export HASHSTACK=/path/to/hashstack/profile
CXX=clang CXXFLAGS="-stdlib=libc++ -I$HASHSTACK/include/c++/v1/" LDFLAGS="-L$HASHSTACK/lib -lc++" cmake ...
```
One has to specify the path to the libc++ headers+libs in <#> explicitly, but otherwise it is working.

TODO:

* [ ] figure out a way to handle the platform specific paths `/usr/include/c++/4.6/` and `/usr/include/c++/4.6/x86_64-linux-gnu/` (see the commit a7d90f84e288b948a4373c18d09ebea821b80cee log for more info)