It was libunistring5.
But to be precise, I had updated Cygwin packages (since I rarely build
on Windows, I do Windows update, Visual Studio Community update, Cygwin
packages update + make clean + autogen.sh) and when I tried to
investigate a bit about this pb, I found something it was about unicode lib.
Then I wanted to search a lib in Cygwin packages and it displayed
"libunistring5" as "waiting for install" or something like this.
I don't understand why it hadn't been installed when I upgraded Cygwin
packages so perhaps there's nothing to add in the wiki unless checking
there's no package in a waiting state.
