Merge bitcoin/bitcoin#27041: Build: Improve handling of suppressed logging in Makefiles

1b1ffbd014b931afb9435ec10911b9a7c130d3e5 Build: Log when test -f fails in Makefile (TheCharlatan)
541012e621386cd824eed81295206a34ba3ba497 Build: Use AM_V_GEN in Makefiles where appropriate (TheCharlatan)

Pull request description:

  This PR triages some behavior around Makefile recipe echoing suppression.

  When generating new files as part of the Makefile the recipe is sometimes suppressed with $(AM_V_GEN) and sometimes with `@`. We should prefer $(AM_V_GEN), since this also prints the lines in silent mode. This is arguably more in style with the current recipe echoing.

  Before:
  `Generated test/data/script_tests.json.h`
  Now:
  `  GEN      test/data/script_tests.json.h`

  A side effect of this change is that the recipe for generating build.h is now echoed on each make run. Arguably this makes its generation more transparent.

  Sometimes the error emitted by `test -f` is currently thrown without any logging. This makes it a bit harder to debug. Instead, print a helpful log message to point the developer in the right direction.

  Alternatively this could have been implemented by just removing the recipe echo suppression (@), but the subsequent make output became too noisy.

ACKs for top commit:
  fanquake:
    ACK 1b1ffbd014b931afb9435ec10911b9a7c130d3e5

Tree-SHA512: e31869fab25e72802b692ce6735f9561912caea903c1577101b64c9cb115c98de01a59300e8ffe7b05b998345c1b64a79226231d7d1453236ac338c62dc9fbb3

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