The zlib code used to be wrapped in a namespace, so it wouldn’t conflict if you linked against zlib. This seems to no longer be the case. Was this an accidental change or deliberate? It is not listed in the breaking changes doc.
This is deliberate, you can use JUCE_INCLUDE_ZLIB_CODE to avoid adding the zlib symbol definitions to the build, and JUCE_ZLIB_INCLUDE_PATH to set the path to the zlib.h that’s already present in your build.
This was changed to allow zlib to build as C instead of C++, which in turn allows us to make fewer changes to the upstream library.