youcompleteme: fix compilation

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.