Cygwin does not have _wopen(), so do not create gzopen_w() there.

madler · madler · commit 5ff989033e8b · 2017-01-16T09:38:36.000-08:00
diff --git a/gzguts.h b/gzguts.h
@@ -39,7 +39,7 @@
 #  include <io.h>
 #endif
 
-#if defined(_WIN32) || defined(__CYGWIN__)
+#if defined(_WIN32)
 #  define WIDECHAR
 #endif
 
diff --git a/zlib.h b/zlib.h
@@ -1893,7 +1893,7 @@ ZEXTERN int            ZEXPORT inflateValidate OF((z_streamp, int));
 ZEXTERN unsigned long  ZEXPORT inflateCodesUsed OF ((z_streamp));
 ZEXTERN int            ZEXPORT inflateResetKeep OF((z_streamp));
 ZEXTERN int            ZEXPORT deflateResetKeep OF((z_streamp));
-#if (defined(_WIN32) || defined(__CYGWIN__)) && !defined(Z_SOLO)
+#if defined(_WIN32) && !defined(Z_SOLO)
 ZEXTERN gzFile         ZEXPORT gzopen_w OF((const wchar_t *path,
                                             const char *mode));
 #endif
