openbabel2: -std=c++14 for code from 2016

fix clang build due to usage of `std::binary_function`

pkgs/development/libraries/openbabel/2.nix

···
 
   buildInputs = [ zlib libxml2 eigen python3 cairo pcre ];
 
+  cmakeFlags = [ "-DCMAKE_CXX_STANDARD=14" ];
+
   nativeBuildInputs = [ cmake pkg-config ];
 
   meta = with lib; {