doc: update unix build doc with build flags #32269

1. 
   bufo24 commented at 6:33 pm on April 14, 2025: contributor

   I had trouble building bitcoin core the way I wanted since now more features require a flag while building. IMO it makes sense to make it a bit more prominent in the build docs how to get the needed flags.

   Related issue: https://github.com/bitcoin/bitcoin/issues/32258

2. doc: update unix doc with build flags 2ca223d282
3. 
   DrahtBot commented at 6:33 pm on April 14, 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/32269.

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   See the guideline for information on the review process. A summary of reviews will appear here.

4. DrahtBot added the label Docs on Apr 14, 2025

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bufo24 DrahtBot maflcko

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