I decided to experiment with the Microsoft Visual Studio 2015 release
candidate. This bundle contains msvc-14.0 for which `_MSC_VER` has the
value 1900.

Some warnings are silenced with this patch to byterun/floats.c

  #if defined (_MSC_VER)
  #  include <float.h>
  #  if(MSC_VER < 1900)
  #    define isnan _isnan
  #    define isfinite _finite
  #  endif/*(MSC_VER <= 1400)*/
  #endif /*defined(_MSC_VER)*/

There is a problem though that comes about here in the production of
'ocamlrun.exe':

link -lib -nologo -machine:AMD64 /out:libcamlrun.lib interp.obj misc.obj
stacks.obj fix_code.obj startup_aux.obj startup.obj freelist.obj
major_gc.obj minor_gc.obj memory.obj alloc.obj roots.obj globroots.obj
fail.obj signals.obj signals_byt.obj printexc.obj backtrace.obj compare.obj
ints.obj floats.obj str.obj array.obj io.obj extern.obj intern.obj hash.obj
sys.obj meta.obj parsing.obj gc_ctrl.obj terminfo.obj md5.obj obj.obj
lexing.obj callback.obj debugger.obj weak.obj compact.obj finalise.obj
custom.obj dynlink.obj win32.obj main.obj
cl -nologo -D_CRT_SECURE_NO_DEPRECATE
-DOCAML_STDLIB_DIR='"C:/ocaml-msvc14/lib"' -I"C:\Program Files
(x86)\flexdll" -O2 -Gy- -MD -c -o prims.obj prims.c
cl : Command line warning D9035 : option 'o' has been deprecated and will
be removed in a future release
prims.c
flexlink -x64 -merge-manifest -stack 33554432 -exe -o ocamlrun.exe
prims.obj ws2_32.lib \
          libcamlrun.lib
** Cannot resolve symbols for descriptor object:
 __iob_func
 sprintf
Makefile.nt:23: recipe for target 'ocamlrun.exe' failed

The second of those missing symbols 'sprintf' can be resolved with this
addition to config/Makefile, line 99

  EXTRALIBS=legacy_stdio_wide_specifiers.lib legacy_stdio_definitions.lib

so that the flexlink step reads

  flexlink -x64 -merge-manifest -stack 33554432 -v -exe -o ocamlrun.exe
prims.obj ws2_32.lib legacy_stdio_wide_specifiers.lib
legacy_stdio_definitions.lib libcamlrun.lib

but the issue with '__iob_func' remains.

I've always associated this sort of problem in the past with mismatch
between /MD and /MT. Research also indicates that this error can manifest
when linking under msvc-14 against objects built with an older version of
the compiler. In any case, after scratching around on it for an hour or
two, I haven't yet found a way to overcome the link failure.

I don't know for sure whether we are looking at a problem with the release
candidate or, a problem with the build system with this new compiler. I do
have a lead or two left to follow - if I have any success at overcoming
this problem, I'll post here.

-- 
Shayne Fletcher