Explanation

See https://discuss.ocaml.org/t/problem-to-compile-a-dll-on-mingw/3661/2
for the short explanation. More in depth discussions:
- the patch that explain why the behaviour changed:
  https://github.com/ocaml/ocaml/pull/1535

- a discussion which allow to understand the problem:
  https://github.com/ocaml/ocaml/issues/6411

I had two choices:
- either use `eval $(ocaml-env cygwin)`
- or add a `-link -static-libgcc` at link time

The first solution works "for me": the PATH environment variable is modified on
my computer, and things are working fine. But since my goal is to distribute the
DLL to others, who do not necessarily have cygwin installed, I chose the second
solution.

dll/dune

@@ -1,7 +1,9 @@
 (executable
   (name tst)
   (libraries tst_stub)
-  (modes shared_object))
+  (modes shared_object)
+  (link_flags :standard -cclib -link -cclib -static-libgcc)
+)
 
 (alias
   (name install)