doc: make -DWITH_ZMQ=ON explicit on build-unix.md #32696

1. 
   luisschwab commented at 10:44 pm on June 6, 2025: contributor

   ZMQ support is not built by default on Linux, and the docs don’t make that clear. This PR makes it explicit that the -DWITH_ZMQ=ON flag is required to build with ZMQ support on build-unix.md.
2. doc: make `-DWITH_ZMQ=ON` explicit on `build-unix.md` 32d4e92b9a
3. 
   DrahtBot commented at 10:44 pm on June 6, 2025: contributor

   The following sections might be updated with supplementary metadata relevant to reviewers and maintainers.

   Code Coverage & Benchmarks

   For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/32696.

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   LLM Linter (✨ experimental)

   Possible typos and grammar issues:

   • “require the following dependency.” -> “require the following dependency:” [uses a period where a colon is needed to introduce the list]

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4. DrahtBot added the label Docs on Jun 6, 2025
5. 
   maflcko commented at 5:59 am on June 7, 2025: member

   lgtm ACK 32d4e92b9ac81858d754487bfec2fef6bed13a57
6. fanquake merged this on Jun 10, 2025
7. fanquake closed this on Jun 10, 2025

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8. fanquake referenced this in commit aea8a394b1 on Jun 10, 2025
9. 
   fanquake commented at 4:29 pm on June 10, 2025: member

   Backported to 29.x in #32589.

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luisschwab DrahtBot maflcko fanquake

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