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/wrapper

@@ -0,0 +1,20 @@
+#!/bin/sh
+
+buildcpath() {
+  local path
+  while (( $# )); do
+    case $1 in
+        -isystem)
+            shift
+            path=$path${path':'}$1
+    esac
+    shift
+  done
+  echo $path
+}
+
+export CPATH=${CPATH}${CPATH:+':'}$(buildcpath ${NIX_CFLAGS_COMPILE})
+export CPATH=${CPATH}${CPATH:+':'}@libc_includes@
+export CPLUS_INCLUDE_PATH=${CPATH}${CPATH:+':'}@libcpp_includes@
+
+exec -a "$0" @clang@/bin/$(basename $0) "$@"