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/Makefile.am

@@ -129,9 +129,11 @@ HUGEIT_0 = @echo "  HUGE    " $@;
 HUGEIT_1 =
 HUGEIT_ = $(HUGEIT_0)
 
+curl_CPPFLAGS = $(AM_CPPFLAGS)
+
 if USE_MANUAL
 # Here are the stuff to create a built-in manual
-AM_CPPFLAGS += -DUSE_MANUAL
+curl_CPPFLAGS += -DUSE_MANUAL
 
 $(ASCIIPAGE):
 	cd $(top_builddir)/docs && $(MAKE)
@@ -167,7 +169,7 @@ CA_EMBED_CSOURCE = tool_ca_embed.c
 curl_cfiles_gen += $(CA_EMBED_CSOURCE)
 CLEANFILES += $(CA_EMBED_CSOURCE)
 if CURL_CA_EMBED_SET
-AM_CPPFLAGS += -DCURL_CA_EMBED
+curl_CPPFLAGS += -DCURL_CA_EMBED
 MK_FILE_EMBED = $(top_srcdir)/src/mk-file-embed.pl
 $(CA_EMBED_CSOURCE): $(MK_FILE_EMBED) $(CURL_CA_EMBED)
 	$(PERL) $(MK_FILE_EMBED) --var curl_ca_embed < $(CURL_CA_EMBED) > $(CA_EMBED_CSOURCE)