Patch needed for zlib to build on Cygwin.  This is needed due to a misguided
and broken attempt to add unnecessary wide character interfaces on Cygwin
(which generally works more on the POSIX philosophy for filenames).
See https://trac.sagemath.org/ticket/23083
diff -ruN a/gzguts.h b/gzguts.h
--- a/gzguts.h	2017-05-26 12:14:03.500413900 +0200
+++ b/gzguts.h	2017-05-26 12:14:28.200295200 +0200
@@ -39,7 +39,7 @@
 #  include <io.h>
 #endif
 
-#if defined(_WIN32) || defined(__CYGWIN__)
+#if defined(_WIN32)
 #  define WIDECHAR
 #endif
 
diff -ruN a/win32/zlib.def b/win32/zlib.def
--- a/win32/zlib.def	2017-05-26 12:14:03.744912800 +0200
+++ b/win32/zlib.def	2017-05-26 12:15:02.512250800 +0200
@@ -91,4 +91,3 @@
     inflateCodesUsed
     inflateResetKeep
     deflateResetKeep
-    gzopen_w
