cc-wrapper: add libcxx for libcxxClang

clang needs to find headers + libraries for compiling with libc++. We
need to add a libcxx argument to cc-wrapper. This means you do not
have to pass in c++ headers directly.

This resolves the last case remaining of #30670. Darwin clang++ now
works properly.

Fixes #30670

pkgs/build-support/cc-wrapper/default.nix

@@ -13,6 +13,7 @@
 , extraPackages ? [], extraBuildCommands ? ""
 , isGNU ? false, isClang ? cc.isClang or false, gnugrep ? null
 , buildPackages ? {}
+, libcxx ? null
 }:
 
 with stdenvNoCC.lib;
@@ -44,8 +45,11 @@ let
   # The wrapper scripts use 'cat' and 'grep', so we may need coreutils.
   coreutils_bin = if nativeTools then "" else getBin coreutils;
 
-  default_cxx_stdlib_compile = optionalString (targetPlatform.isLinux && !(cc.isGNU or false) && !nativeTools && cc ? gcc)
-    "-isystem $(echo -n ${cc.gcc}/include/c++/*) -isystem $(echo -n ${cc.gcc}/include/c++/*)/$(${cc.gcc}/bin/gcc -dumpmachine)";
+  default_cxx_stdlib_compile = if (targetPlatform.isLinux && !(cc.isGNU or false) && !nativeTools && cc ? gcc) then
+    "-isystem $(echo -n ${cc.gcc}/include/c++/*) -isystem $(echo -n ${cc.gcc}/include/c++/*)/$(${cc.gcc}/bin/gcc -dumpmachine)"
+  else if targetPlatform.isDarwin && (libcxx != null) && (cc.isClang or false) then
+    "-isystem ${libcxx}/include/c++/v1"
+  else "";
 
   # The "infix salt" is a arbitrary string added in the middle of env vars
   # defined by cc-wrapper's hooks so that multiple cc-wrappers can be used
@@ -256,9 +260,9 @@ stdenv.mkDerivation {
 
       echo "$ccLDFlags" > $out/nix-support/cc-ldflags
       echo "$ccCFlags" > $out/nix-support/cc-cflags
-    ''
-
-    + optionalString propagateDoc ''
+    '' + optionalString (targetPlatform.isDarwin && (libcxx != null) && (cc.isClang or false)) ''
+      echo " -L${libcxx}/lib" >> $out/nix-support/cc-ldflags
+    '' + optionalString propagateDoc ''
       ##
       ## Man page and info support
       ##

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

@@ -59,6 +59,7 @@ let
 
     libcxxClang = wrapCCWith rec {
       cc = tools.clang-unwrapped;
+      libcxx = targetLlvmLibraries.libcxx;
       extraPackages = [
         targetLlvmLibraries.libcxx
         targetLlvmLibraries.libcxxabi

pkgs/development/compilers/llvm/6/default.nix

@@ -59,6 +59,7 @@ let
 
     libcxxClang = wrapCCWith rec {
       cc = tools.clang-unwrapped;
+      libcxx = targetLlvmLibraries.libcxx;
       extraPackages = [
         targetLlvmLibraries.libcxx
         targetLlvmLibraries.libcxxabi

pkgs/development/compilers/llvm/7/default.nix

@@ -59,6 +59,7 @@ let
 
     libcxxClang = wrapCCWith rec {
       cc = tools.clang-unwrapped;
+      libcxx = targetLlvmLibraries.libcxx;
       extraPackages = [
         targetLlvmLibraries.libcxx
         targetLlvmLibraries.libcxxabi