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 0035ff45c5 #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 137aecfbf1 #13514
  Follow-up to 541321507e #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 25119fbaaa #18104

Closes #18118

.github/workflows/distcheck.yml

@@ -113,7 +113,7 @@ jobs:
           make test-ci
           make install
           curl-install/bin/curl --disable --version
-          curl-install/bin/curl --manual | wc -l
+          curl-install/bin/curl --manual | wc -l | grep -v '^ *0$'
           popd
           scripts/checksrc-all.pl
 
@@ -127,9 +127,6 @@ jobs:
         with:
           name: 'release-tgz'
 
-      - name: 'disable preinstalled perl'
-        run: sudo mv "$(command -v perl)" "$(command -v perl)"-disabled
-
       - name: 'build & install'
         run: |
           echo "::stop-commands::$(uuidgen)"
@@ -137,11 +134,11 @@ jobs:
           pushd curl-99.98.97
           mkdir build
           pushd build
-          ../configure --prefix="$PWD"/curl-install --enable-werror --without-ssl --without-libpsl --disable-docs
+          ../configure --prefix="$PWD"/curl-install --enable-werror --without-ssl --without-libpsl ac_cv_path_PERL=
           make
           make install
           curl-install/bin/curl --disable --version
-          curl-install/bin/curl --manual | wc -l
+          curl-install/bin/curl --manual | wc -l | grep -v '^ *0$'
           popd
 
   verify-in-tree-autotools:
@@ -154,19 +151,16 @@ jobs:
         with:
           name: 'release-tgz'
 
-      - name: 'disable preinstalled perl'
-        run: sudo mv "$(command -v perl)" "$(command -v perl)"-disabled
-
       - name: 'build & install'
         run: |
           echo "::stop-commands::$(uuidgen)"
           tar xvf curl-99.98.97.tar.gz
           pushd curl-99.98.97
-          ./configure --prefix="$PWD"/curl-install --enable-werror --without-ssl --without-libpsl --disable-docs
+          ./configure --prefix="$PWD"/curl-install --enable-werror --without-ssl --without-libpsl ac_cv_path_PERL=
           make
           make install
           curl-install/bin/curl --disable --version
-          curl-install/bin/curl --manual | wc -l
+          curl-install/bin/curl --manual | wc -l | grep -v '^ *0$'
 
   verify-out-of-tree-cmake:
     name: 'CM out-of-tree !perl'
@@ -188,7 +182,7 @@ jobs:
           cmake --install build
           export LD_LIBRARY_PATH="$PWD/curl-install/lib:$LD_LIBRARY_PATH"
           curl-install/bin/curl --disable --version
-          curl-install/bin/curl --manual | wc -l
+          curl-install/bin/curl --manual | wc -l | grep -v '^ *0$'
 
   verify-in-tree-cmake:
     name: 'CM in-tree !perl'
@@ -210,7 +204,7 @@ jobs:
           cmake --install .
           export LD_LIBRARY_PATH="$PWD/curl-install/lib:$LD_LIBRARY_PATH"
           curl-install/bin/curl --disable --version
-          curl-install/bin/curl --manual | wc -l
+          curl-install/bin/curl --manual | wc -l | grep -v '^ *0$'
 
   missing-files:
     name: 'missing files'