anchor.diff:  FlexDLL and OCaml are given <<seflexdll,later in this document>>.  Unless you
   bootstrap FlexDLL, you will need to ensure that the directory to which you
   install FlexDLL is included in your `PATH` environment variable. Note: if you
  -use Visual Studio 2015, the binary distribution of FlexDLL will not work and
  -you must build it from sources.
  +use Visual Studio 2015 or Visual Studio 2017, the binary distribution of
  +FlexDLL will not work and you must build it from sources.
   
   The base bytecode system (ocamlc, ocaml, ocamllex, ocamlyacc, ...) of all three
   ports runs without any additional tools.
  
positive.code: 
  for 64-bit. For Visual Studio 2005-2013, you need to use one of the shortcuts in
  the "Visual Studio Tools" program group under the main program group for the
  version of Visual Studio you installed. For Visual Studio 2015, you need to use
  the shortcuts in the "Windows Desktop Command Prompts" group under the
  "Visual Studio Tools" group.
  
  Unlike `SetEnv` for the Windows SDK, the architecture is selected by using a
  different shortcut, rather than by running a command.
  
negative.code: 
  For Visual Studio 2012 and 2013, both x86 and x64 Command Prompt shortcuts
  indicate if they are the "Native Tools" or "Cross Tools" versions. Visual Studio
  2015 and 2017 make the shortcuts even clearer by including the full name of the
  architecture.
  
  You cannot at present use a cross-compiler to compile 64-bit OCaml on 32-bit
  