clang: fix install on Darwin

As of Nix 2.0, building the `user-environment` package on macOS (Darwin)
fails because LLVMgold.so is a broken symlink. Fix the issue by not
creating the symlink in the first place, since it wouldn't be used on
Darwin anyway.

pkgs/development/compilers/llvm/5/clang/default.nix

@@ -52,7 +52,9 @@ let
 
     postInstall = ''
       # Clang expects to find LLVMgold in its own prefix
+      if [ -e ${llvm}/lib/LLVMgold.so ]; then
         ln -sv ${llvm}/lib/LLVMgold.so $out/lib
+      fi
       # Clang expects to find sanitizer libraries in its own prefix
       ln -sv ${llvm}/lib/clang/${release_version}/lib $out/lib/clang/${release_version}/
       ln -sv $out/bin/clang $out/bin/cpp