add gcc include path in clangd config

[WilliamHsieh]

• precompile header <bits/stdc++.h>
• g++ -E -P / cpp -P / cpp -H > /dev/null
• somehow fit shell output inside home.*.file

  "clangd".text = (builtins.readFile ./config.nix) + ''
    ${pkgs.gcc}/bin/cpp -v -x c++ /dev/null 2>&1 | sed -n -e "/#include <...>/,/End/p" | sed '1d;$d' | sed 's/ /    - -I/'
  '';

https://stackoverflow.com/questions/4980819/what-are-the-gcc-default-include-directories

[WilliamHsieh]

https://clangd.llvm.org/guides/system-headers#query-driver