build: if no perl, fix to use the pre-built hugehelp, if present

[vszakats]

• cmake: Before this patch a missing perl disabled the curl manual.
  After this patch, it automatically picks up a pre-built hugehelp,
  if present (= when building from a release tarball).
  Follow-up to 0035ff4 build: drop tool_hugehelp.c.cvs, tidy up macros, drop buildconf.bat #16081

• autotools: Rework behavior when perl is missing. Before this patch
  it caused a hard error when docs/manual/ca-embed were enabled.
  Of these, docs were enabled by default. After this patch, doc
  generation is automatically skipped, with a warning. Manual generation
  falls back to using a pre-built hugehelp, or a stub if that's missing.
  CA-embed is automatically skipped, with a warning.
  Slight difference from cmake: When built with no perl and no pre-built
  hugehelp, the manual is enabled, but the content is empty; with cmake
  it's disabled proper.
  Follow-up to 137aecf configure: error on missing perl if docs or manual is enabled #13514
  Follow-up to 5413215 configure: add --disable-docs flag #12857

Also:

• GHA/distcheck: verify if the pre-built hugehelp makes it into curl.
• GHA/distcheck: streamline disabling perl with autotools.
• GHA/distcheck: --disable-docs no longer required to build without perl.
  Passing it also implicitly disables the curl manual, which is
  undesired in these tests.
• cmake: drop redundant HAVE_MANUAL_TOOLS interim variable.
• cmake: move two set() lines close to their counterparts.

Follow-up to 25119fb #18104

---

w/o ws https://github.com/curl/curl/pull/18118/files?w=1

• autotools: also try fixing.

Tested OK in CI:
https://github.com/curl/curl/actions/runs/16671747516?pr=18118
also locally using 8 combinations (with autotools).