On native builds `llvm-config` picks up `zlib` and this gets pased into
the rust build tools, but on cross builds `llvm-config` is explicitly
ignored as it contains information for the host system and cannot be
trusted to be accurate for the target system.
Both DragonFly and Solaris contain `zlib` in the base system, so this is
both a safe assumption and required for a successful cross build unless
`zlib` support is disabled in LLVM.
This is more or less in the same vein as #75713 and #75655.
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