libc++abi-dev lacks include

[Axel Huebl]

Even after installing libc++-dev and libc++abi-dev, clang can not compile with `-stdlib=libc++` if it needs to include `<cxxabi.h>`.

The reason lies in a missing include or symlink in cxxabi.h
```
/usr/include/c++/v1/cxxabi.h:21:10: fatal error: '__cxxabi_config.h' file not
      found
#include <__cxxabi_config.h>
         ^~~~~~~~~~~~~~~~~~~
1 error generated.
```

That file is located in /usr/include/libcxxabi/

Maybe add a symlink or install it in a different, clang-findable location?