cygwin used to be fully supported, but support broke because there was no one around that was maintaining it.  There has been renewed interest, and some progress toward fixing cygwin support (more help is appreciated), but it cannot be called a supported platform at the moment because there is no maintainer on our team and there is no buildbot.  There are hopes that both of these deficiencies will be remedied.

This issue is a duplicate of issue 21085, and there is a patch in that issue that has not been applied yet.