lwt.5.9.2 doesn't compile anymore on Windows with the Cygwin backend

[kit-ty-kate]

Looks like some assumption about the linker were made that doesn't hold on Cygwin.

#=== ERROR while compiling lwt.5.9.2 ==========================================#
# context              2.5.0~alpha1~dev | cygwin/x86_64 | ocaml-system.4.14.2 | git+https://github.com/ocaml/opam-repository.git#38a1469dbbc69c770e534e26f79c97256a442b71
# path                 ~/.opam/default/.opam-switch/build/lwt.5.9.2
# command              ~/.opam/default/bin/dune build -p lwt -j 3 @install
# exit-code            1
# env-file             ~/.opam/log/lwt-623-ad3ec6.env
# output-file          ~/.opam/log/lwt-623-ad3ec6.out
### output ###
# Usage:
# [...]
# 
# Notes:
# * The -I, -l and -L options do not need to be separated from their argument.
# * An option like /linkXXX is an abbrevation for '-link XXX'.
# * An option like -Wl,-XXX is an abbreviation for '-link -XXX'.
# * FlexDLL's object files are searched by default in the same directory as
#   flexlink, or in the directory given by the environment variable FLEXDIR
#   if it is defined.
# * Extra argument can be passed in the environment variable FLEXLINKFLAGS.
# 
# Homepage: http://alain.frisch.fr/flexdll.html