cc-wrapper: externalize default_cxx_stdlib_compile

This value is require to get c++ std include path for libclang based tools (vim plugins in my case).
I currently extract it this with this rather command:

```
eval echo $(nix-instantiate --eval --expr 'with (import <nixpkgs>) {}; clang.default_cxx_stdlib_compile')
```

it did not trigger any recompilation on my system.