zlib: Use unistd.h on all non-Windows platforms

Replace the `HAVE_UNISTD_H` change from commit 3edcd70754 (ENH: Update
zlib to 1.2.3, 2007-09-11, v2.6.0~1135) with simpler logic.

Utilities/cmzlib/zconf.h

@@ -284,7 +284,7 @@ typedef uLong FAR uLongf;
    typedef Byte       *voidp;
 #endif
 
-#if 0           /* HAVE_UNISTD_H -- this line is updated by ./configure */
+#if !defined(_WIN32)
 #  include <sys/types.h> /* for off_t */
 #  include <unistd.h>    /* for SEEK_* and off_t */
 #  ifdef VMS