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

src/tool_operate.c

@@ -94,9 +94,7 @@
 #include "tool_xattr.h"
 #include "tool_vms.h"
 #include "tool_help.h"
-#ifndef UNITTESTS
 #include "tool_hugehelp.h"
-#endif
 #include "tool_progress.h"
 #include "tool_ipfs.h"
 #include "dynbuf.h"
@@ -107,10 +105,6 @@ CURL_EXTERN CURLcode curl_easy_perform_ev(CURL *easy);
 
 #include "memdebug.h" /* keep this as LAST include */
 
-#ifdef UNITTESTS
-#undef CURL_CA_EMBED
-#endif
-
 #ifdef CURL_CA_EMBED
 #ifndef CURL_DECLARED_CURL_CA_EMBED
 #define CURL_DECLARED_CURL_CA_EMBED
@@ -3191,8 +3185,10 @@ CURLcode operate(struct GlobalConfig *global, int argc, argv_item_t argv[])
         tool_help(global->help_category);
       /* Check if we were asked for the manual */
       else if(res == PARAM_MANUAL_REQUESTED) {
-#ifndef UNITTESTS
+#ifdef USE_MANUAL
         hugehelp();
+#else
+        puts("built-in manual was disabled at build-time");
 #endif
       }
       /* Check if we were asked for the version information */