adapt to coq/coq#15220

I'd like to do something like

https://github.com/mattam82/Coq-Equations/pull/459/files#diff-76ed074a9305c04054cdebb9e9aad2d818052b07091de1f20cad0bbac34ffb52

but I could not do it in remake, help is appreciated. I suspect the current code does not work well on cygwin.