rust-bindgen: wrap to add required library compilation flags

The easy part is to add NIX_CFLAGS_COMPILE for "regular" libraries.
A bit more tricky is to add the required flags for libclang to find
libstdcxx. For this we parse arguments to bindgen to look for
-x c++ or -xc++ and if found add NIX_CXXSTDLIB_COMPILE to the arguments.
This variable is populated by a complex dance of setupHooks. We trigger
this by adding clang to propagatedBuildInputs. A more subtle way may
exist.

pkgs/development/tools/rust/bindgen/wrapper.sh

@@ -0,0 +1,36 @@
+#!/usr/bin/env bash
+sep='--'   # whether to add -- before new options
+cxx=0      # whether cxx was explicitly requested
+lastWasx=0 # whether the last argument passed was -x
+for e in "$@"; do
+  if [[ "$e" == "--" ]]; then
+    sep=
+  fi;
+  if [[ "$sep" == "" ]]; then
+    # we look for -x c++ after -- only
+    if [[ "$e" == "-x" ]]; then
+      lastWasx=1
+    fi;
+    if [[ $lastWasx -eq 1 && "$e" == "c++" ]]; then
+      lastWasx=0
+      cxx=1
+    fi;
+    if [[ "$e" == "-xc++" || "$e" == -std=c++* ]]; then
+      cxx=1
+    fi;
+  fi;
+done;
+cxxflags=
+if [[ $cxx -eq 1 ]]; then
+  cxxflags=$NIX_CXXSTDLIB_COMPILE
+fi;
+if [[ -n "$NIX_DEBUG" ]]; then
+  set -x;
+fi;
+export LIBCLANG_PATH="@libclang@/lib"
+# shellcheck disable=SC2086
+# cxxflags and NIX_CFLAGS_COMPILE should be word-split
+exec -a "$0" @out@/bin/.bindgen-wrapped "$@" $sep $cxxflags $NIX_CFLAGS_COMPILE
+# note that we add the flags after $@ which is incorrect. This is only for the sake
+# of simplicity.
+