root5: broken with clang

Seems like any clang version > 5 doesn't work. Since root >= 6.0 is
available from nixpkgs, I think it is reasonable to mark it as broken
for now. Maybe someone can figure out if it is possible to backport a
fix from root >= 6.0.

pkgs/applications/science/misc/root/5.nix

···
     homepage = "https://root.cern.ch/";
     description = "A data analysis framework";
     platforms = platforms.unix;
-    broken = !stdenv.isx86_64;
+    broken = !stdenv.isx86_64 || stdenv.cc.isClang or false;
     maintainers = with maintainers; [ veprbl ];
     license = licenses.lgpl21;
   };

pkgs/top-level/all-packages.nix

···
 
   root5 = lowPrio (callPackage ../applications/science/misc/root/5.nix {
     inherit (darwin.apple_sdk.frameworks) Cocoa OpenGL;
-    stdenv = if stdenv.cc.isClang then llvmPackages_7.stdenv else stdenv;
   });
 
   rinetd = callPackage ../servers/rinetd { };