Error "The command line is too long" for very short command line (<250 characters) on Cygwin/Windows

Dear Dune Team,

In the Windows build for INRIA Coq I use dune to build e.g. lablgtk. Since a short while, possibly since switching to OCaml 4.08 (from 4.07) I get strange errors about long command lines. e.g.:

lablgtk-3.0.beta5-dune-build_err.txt: (cd _build/default && C:\bin\cygwin_coq64_ltac2_array\usr\x86_64-w64-mingw32\sys-root\mingw\bin\ocamlopt.opt.exe -w -40 -w -6-7-9-10-27-32-33-34-35-36-50-52 -no-strict-sequence -g -shared -linkall -I src -o src/lablgtk3.cmxs src/lablgtk3.cmxa)
lablgtk-3.0.beta5-dune-build_err.txt: The command line is too long.

The build does suceed if I reduce the path by one character from C:\bin\cygwin_coq64_ltac2_array to C:\bin\cygwin_coq64_ltac2_arra. This is reproducible.

I am quite sure that I did use much longer path names recently. The only thing I know has been changed is the switch to OCaml 4.08, but of cause Coq is a large project with many contributors. If this helps I can provide one click batch files which setup cygwin and build for the good and for the bad case on Windows. In case you think it could indeed be the switch to OCaml 4.08 I could also retest with OCaml 4.07 (it is a bit of work cause of the substantially different build procedures for OCaml 4.07 and 4.08 on Windows).

Btw.: I tested this with dune 2.0.0 and dune 2.1.0.

Reproduce the reported build on Cygwin/Windows using the short and one-character-shorter paths from the issue, with Dune 2.0.0 or 2.1.0. Compare the behavior across the reported OCaml 4.08 and 4.07 environments; done means the reproducible short command no longer fails as too long.