Add flexlink issue

Author: jmid

README.md

@@ -390,6 +390,13 @@ property can be done in two different ways:
 Issues
 ======
 
+Regression causing a Cygwin configure to fail (new, configure)
+--------------------------------------------------------------
+
+A configure PR accidentally [introduced a regression causing a flexlink test to
+fail for a Cygwin build](https://github.com/ocaml/ocaml/pull/13009)
+
+
 Regression on output to closed `Out_channel`s (new, fixed, runtime)
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