opentype-sanitizer: build with C++17 stdlib

gtest enforces C++17 now: https://github.com/NixOS/nixpkgs/pull/403142

pkgs/by-name/op/opentype-sanitizer/package.nix

···
     hash = "sha256-QRbF2GUDQsp8i6qVYlafSb9HaaozRuJ8dn1mhMMLeLc=";
   };
 
-  mesonFlags = [ "-Dcpp_std=c++14" ];
+  mesonFlags = [ "-Dcpp_std=c++17" ]; # required by gtest
 
   buildInputs = [
     freetype