Log Message:
libzip: update to 1.5.1.

1.5.1 [2018-04-11]
==================

* Choose format of installed documentation based on available tools.
* Fix visibility of symbols.
* Fix zipcmp directory support.
* Don't set RPATH on Linux.
* Use Libs.private for link dependencies in pkg-config file.
* Fix build with LibreSSL.
* Various bugfixes.