Add optional zlib support detection in extconf

[gemmaro]

Hello,

This adds detection for optional zlib support that may be available indirectly through libxml2, to extconf.rb.

Background:
While this gem doesn't directly depend on zlib, it can leverage zlib functionality when available through libxml2. Currently, the HAVE_ZLIB_H macro is never defined, which means zlib support is always disabled even when zlib is actually available via libxml2. This change allows the extension to conditionally use zlib features.

Thank you.

[cfis]

Thanks!

[cfis]

I should have paid more attention, looks like this commit broke windows. Can you fix?

See e50f31a

[gemmaro]

Thanks for bringing this to my attention. I suspected the Windows CI might have been broken before this change, so I did some testing to verify.

I created a test branch and pull request to isolate the issue:

• Test PR: enable workflow dispatch for CI test purpose gemmaro/libxml-ruby#1
• Commit history: https://github.com/gemmaro/libxml-ruby/commits/draft/windows/ci/

The CI fails the same way, confirming this is an existing Windows CI issue unrelated to the zlib detection changes. This is probably an issue that emerged over time due to ecosystem changes - it likely worked fine in the past.

I have access to a Windows environment and am trying to investigate and fix this, but I'm struggling to set up the compilation environment for this gem. Since it might take some time to resolve, it would probably be better to create a separate issue for this.

[cfis]

Actually it looks like this - aed5b20

I have a windows build environment setup already, so can take a look.