build: drop tool_hugehelp.c.cvs, tidy up macros, drop buildconf.bat

Rework the way `tool_hugehelp.c` is included in builds.

After this patch, with `./configure` and CMake `tool_hugehelp.c` is only
compiled when building with manuals enabled. With manuals disabled this
source file is not used anymore. The method is similar to how
8a3740bc8e implemented `tool_ca_embed.c`.
`./configure` always generates it as before, otherwise the build fails.

- winbuild: rework to not need `buildconf.bat`, but automatically use
  `tool_hugehelp.c` if present (e.g. when building from an official
  source tarball) and enable `USE_MANUAL` accordingly.

- `buildconf.bat`: after dropping `tool_hugehelp.c` generation, the only
  logic left was `cp Makefile.dist Makefile`. This allowed to launch
  winbuild builds via GNU Make in a Git repo. Drop this option together
  with the batch file.

- build `libcurltool` without `USE_MANUAL` macro to exclude the manual
  and the dependence on the generator commands. Drop relying on
  `UNITTESTS` for this purpose.
  Follow-up to 96843f4ef7 #16068

- `src/mkhelp.pl`: include `tool_hugehelp.h` before using `USE_MANUAL`
  to have it set in `config-*.h` builds with source tarballs created
  with manual but without zlib.

Closes #16081

tests/testcurl.pl

@@ -566,7 +566,6 @@ if ($configurebuild) {
   logit "copying files to build dir ...";
   if ($^O eq 'MSWin32') {
     system("xcopy /s /q \"$CURLDIR\" .");
-    system("buildconf.bat");
   }
   elsif ($^O eq 'linux') {
     system("cp -afr $CURLDIR/* .");