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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4. DrahtBot added the label Docs on Apr 14, 2025
5. in doc/build-unix.md:15 in 2ca223d282

    6@@ -7,8 +7,15 @@ Some notes on how to build Bitcoin Core in Unix.
    7 To Build
    8 ---------------------
    9 
   10+### 1. Configuration
   11+
   12 ```bash
   13 cmake -B build
   14+```
   15+Run `cmake -B build -LH` to see the full list of available options.
   

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   maflcko commented at 8:13 am on April 15, 2025:

   It doesn’t seem to be unix related, but I don’t have a different suggestion.

   You could also mention something along the lines

   The CMake variables have fixed default values and are not auto-detected at runtime. Any CMake build that requires values different from the defaults must explicitly set them. CMake will print a configure summary, which can be used to verify the build configuration.

   However, this is also not unix specific :shrug:

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   bufo24 commented at 8:55 am on April 15, 2025:

   I noticed that this is shown in other build docs except for the unix one, like the macos one here: https://github.com/bitcoin/bitcoin/blob/master/doc/build-osx.md#further-configuration. That’s why I added this more higher up in the doc for unix

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   hebasto commented at 8:39 pm on April 15, 2025:

   I noticed that this is shown in other build docs except for the unix one…

   https://github.com/bitcoin/bitcoin/blob/cdc32994feadf3f15df3cfac5baae36b4b011462/doc/build-unix.md?plain=1#L178-L183

6. DrahtBot added the label CI failed on Apr 28, 2025
7. DrahtBot removed the label CI failed on May 2, 2025

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