Solving issue https://github.com/javer/gentoo-overlay/issues/16

by having hhvm not build its own libzip.

Actually hhvm shouldn't install any system libs. It's fine (kinda) to build its own libs, but then they should either be installed somewhere else or linked statically.
In the meantime this fixes the build, since hhvm package depends on libzip, but then tries to install its own, making it impossible to build. We could use the same method for lz4 I guess (add lz4 to dependencies, disable its building by hhvm).