[ci skip] Note the required configure changes for bundled libzip

UPGRADING

@@ -477,6 +477,10 @@ PCRE:
     (`(object) array( ... )`), rather than using the nonexistent method
     stdClass::__setState().
 
+ Zip:
+  . Building against the bundled libzip is discouraged, but still possible by
+    adding `--without-libzip` to the configuration.
+
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 10. New Global Constants
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