docs: Add snippet for zlib system dependency

docs/markdown/snippets/zlib_system_dependency.md

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+## Add a system type dependency for zlib
+
+This allows zlib to be detected on macOS and FreeBSD without the use of
+pkg-config or cmake, neither of which are part of the base install on those
+OSes (but zlib is).
+
+A side effect of this change is that `dependency('zlib')` also works with
+cmake instead of requiring `dependency('ZLIB')`.