Don't install bundled libzip

Apart from header installation, which is done by
third-party/CMakeFiles.txt, installation of the bundled libzip is not
necessary, since it is statically linked. In fact, installation can
cause a previously installed version to be used in preference to the
version in the source tree, which is harmful in itself. It can also mess
up the -I flags for the rest of HHVM; see
https://github.com/facebook/hhvm/issues/3089 for details.
Merge pull request #16 from tstarling/dont-install-libzip

Don't install bundled libzip