clang-tools: teach about nix's include path

By translating NIX_CFLAGS_COMPILE to CPATH,
all tools will now find c headers properly,
when run in a nix-shell.

pkgs/development/tools/clang-tools/default.nix

@@ -13,15 +13,20 @@ in stdenv.mkDerivation {
     runHook preInstall
 
     mkdir -p $out/bin
+    export libc_includes="${stdenv.lib.getDev stdenv.cc.libc}/include"
+    export libcpp_includes="${llvmPackages.libcxx}/include/c++/v1"
+
+    export clang=${clang}
+    substituteAll ${./wrapper} $out/bin/clangd
+    chmod +x $out/bin/clangd
     for tool in \
       clang-apply-replacements \
       clang-check \
       clang-format \
       clang-rename \
-      clang-tidy \
-      clangd
+      clang-tidy
     do
-      ln -s ${clang}/bin/$tool $out/bin/$tool
+      ln -s $out/bin/clangd $out/bin/$tool
     done
 
     runHook postInstall