root: re-enable unvendored LLVM

pkgs/applications/science/misc/root/default.nix

@@ -1,6 +1,7 @@
 { stdenv
 , lib
 , fetchurl
+, fetchpatch
 , makeWrapper
 , cmake
 , git
@@ -15,6 +16,7 @@
 , libGLU
 , libGL
 , libxml2
+, llvm_9
 , lz4
 , xz
 , pcre
@@ -53,6 +55,7 @@ stdenv.mkDerivation rec {
     zlib
     zstd
     libxml2
+    llvm_9
     lz4
     xz
     gsl
@@ -71,8 +74,23 @@ stdenv.mkDerivation rec {
 
   patches = [
     ./sw_vers.patch
+
+    # Fix builtin_llvm=OFF support
+    (fetchpatch {
+      url = "https://github.com/root-project/root/commit/0cddef5d3562a89fe254e0036bb7d5ca8a5d34d2.diff";
+      excludes = [ "interpreter/cling/tools/plugins/clad/CMakeLists.txt" ];
+      sha256 = "sha256-VxWUbxRHB3O6tERFQdbGI7ypDAZD3sjSi+PYfu1OAbM=";
+    })
   ];
 
+  # Fix build against vanilla LLVM 9
+  postPatch = ''
+    sed \
+      -e '/#include "llvm.*RTDyldObjectLinkingLayer.h"/i#define private protected' \
+      -e '/#include "llvm.*RTDyldObjectLinkingLayer.h"/a#undef private' \
+      -i interpreter/cling/lib/Interpreter/IncrementalJIT.h
+  '';
+
   preConfigure = ''
     rm -rf builtins/*
     substituteInPlace cmake/modules/SearchInstalledSoftware.cmake \
@@ -99,6 +117,7 @@ stdenv.mkDerivation rec {
     "-DCMAKE_CXX_STANDARD=17"
     "-DCMAKE_INSTALL_LIBDIR=lib"
     "-DCMAKE_INSTALL_INCLUDEDIR=include"
+    "-Dbuiltin_llvm=OFF"
     "-Dbuiltin_nlohmannjson=OFF"
     "-Dbuiltin_openui5=OFF"
     "-Dalien=OFF"