Cygwin is POSIX, don't say it isn't
More to the point, Cygwin is a POSIX API.  In our library, the use of
a POSIX API is marked by defining the macro OPENSSL_SYS_UNIX.
Therefore, that macro shouldn't be undefined when building for Cygwin.
@@ -49,7 +49,6 @@ extern "C" {
# define OPENSSL_SYS_WIN32_UWIN
# else
# if defined(__CYGWIN__) || defined(OPENSSL_SYS_CYGWIN)
-# undef OPENSSL_SYS_UNIX
# define OPENSSL_SYS_WIN32_CYGWIN
# else
# if defined(_WIN32) || defined(OPENSSL_SYS_WIN32)
