I believe zlib was added to https://github.com/LLNL/benchpark/tree/develop/systems/genericx86/externals/base/00-packages.yaml to make Laghos work.

* [ ]  What was the reason?  Is this because we need the handle?

This was to avoid spack from defaulting to building zlib-ng, which does not work for non-intel compilers

* [ ]  Is the correct solution to put it in externals for each system?

Possibly. But that will assume every system has to have zlib installed

* [ ]  If yes, does every system definition have to have it? 
* [ ]  If yes, is there a better place to put it?

The Spack team may be able to clarify this better

After discussion 11/26, we should remove zlib from packages on all systems except for lassen. @rfhaque.