Merge bitcoin/bitcoin#23593: build: remove x-prefix's from comparisons

d6d402bd2b8211e9e111acae433ca4e3cfd8d370 build: remove x-prefix comparisons (fanquake)

Pull request description:

  Very old shells suffered from bugs which meant that prefixing variables
  with an "x" to ensure that the lefthand side of a comparison always
  started with an alphanumeric character was needed. Modern shells don't
  suffer from this issue (i.e Bash was fixed in 1996).

  In any case, we've already got unprefixed checks used in our codebase, i.e
  https://github.com/bitcoin/bitcoin/blob/681b25e3cd7d084f642693152322ed9a40f33ba0/configure.ac#L292
  and have libs (in depends) that also use unprefixed comparisons in their
  configure scripts.

  I think it's time that we consolidate on not using the x-prefix workaround.
  At best it's mostly just confusing. Could simplify some of these checks
  further in future.

  More info:
  https://github.com/koalaman/shellcheck/wiki/SC2268
  https://www.vidarholen.net/contents/blog/?p=1035

ACKs for top commit:
  MarcoFalke:
    Concept ACK d6d402bd2b8211e9e111acae433ca4e3cfd8d370

Tree-SHA512: 70030d61dcdb5b009823d83d73204627de53d2f628d8d6478e8e16804ac09f6bbdc53cbb1a6fb2085ebfe1a694b576e46ff381fb980cf667fab4bbadc79587d7

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