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.
1) Replace --system-libclang with an explicit setting for the path to
nix libclang 3.9. Otherwise it will find system's libclang on darwin.
2) Remove clang/llvm from build inputs. This prevents ycm itself from
being compiled by clang 3.9. stdenv's standard compiler (gcc or
clang 3.7) will suffice.
3) Enable C++11 again as #16212 is caused by incopatibility between
clang 3.9 and libc++ 3.7, but we don't compile YCM with clang 3.9, so
this is not needed anymore.
