C++ Compilers: Systematize handling of standard libraries

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

@@ -1,4 +1,4 @@
-{ lowPrio, newScope, pkgs, stdenv, cmake, gcc, libstdcxxHook
+{ lowPrio, newScope, pkgs, stdenv, cmake
 , libxml2, python3, isl, fetchurl, overrideCC, wrapCCWith
 , buildLlvmTools # tools, but from the previous stage, for cross
 , targetLlvmLibraries # libraries, but from the next stage, for cross
@@ -51,25 +51,17 @@ let
 
     libstdcxxClang = wrapCCWith rec {
       cc = tools.clang-unwrapped;
-      extraTools = [
-        libstdcxxHook
-      ];
+      libcxx = null; # libstdcxx is smuggled in with clang.gcc
       extraPackages = [
         targetLlvmLibraries.compiler-rt
       ];
-      extraBuildCommands = ''
-          echo "-target ${targetConfig}" >> $out/nix-support/cc-cflags
-          echo "-B${gcc.cc}/lib/gcc/${targetConfig}/${gcc.version}" >> $out/nix-support/cc-cflags
-          echo "-L${gcc.cc}/lib/gcc/${targetConfig}/${gcc.version}" >> $out/nix-support/cc-ldflags
-          echo "-L${gcc.cc.lib}/${targetConfig}/lib" >> $out/nix-support/cc-ldflags
-      '' + mkExtraBuildCommands cc;
+      extraBuildCommands = mkExtraBuildCommands cc;
     };
 
     libcxxClang = wrapCCWith rec {
       cc = tools.clang-unwrapped;
       libcxx = targetLlvmLibraries.libcxx;
       extraPackages = [
-        targetLlvmLibraries.libcxx
         targetLlvmLibraries.libcxxabi
         targetLlvmLibraries.compiler-rt
       ];