treewide: disable -Werror for llvm 7

Some of these have errors on newest llvm. It’s easiest to just add
-Wno-error in these cases.

pkgs/tools/archivers/p7zip/default.nix

@@ -37,6 +37,8 @@ stdenv.mkDerivation rec {
 
   setupHook = ./setup-hook.sh;
 
+  NIX_CFLAGS_COMPILE = "-Wno-error=c++11-narrowing";
+
   meta = {
     homepage = http://p7zip.sourceforge.net/;
     description = "A port of the 7-zip archiver";