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

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

- 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 0035ff45c59437db1a8ffe960b40905c8582b149 #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 137aecfbf1e933e7228a4f62cc977eabffb2afbf #13514
  Follow-up to 541321507e386744c3ea1200cc83cc98338e27d7 #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 25119fbaaa76a182d4b08d64853d0a8db9181219 #18104

Closes #18118