Cygwin has uselocale() etc. since version 2.6. See
https://cygwin.com/ml/cygwin-announce/2016-08/msg00095.html
Let me update the documentation.
2018-12-18 Bruno Haible
localename: Update comments regarding Cygwin.
* lib/localename.c: Update comment.
* doc/posix
o Haible
localename: Update comments.
* lib/localename.c (HAVE_USELOCALE): Update list of platforms.
diff --git a/lib/localename.c b/lib/localename.c
index 3fddbf9..aa3cc13 100644
--- a/lib/localename.c
+++ b/lib/localename.c
@@ -2623,7 +2623,8 @@ get_lcid (const char *locale_name)
#endif