Makefile.mk: portable Makefile.m32

Update bare GNU Make `Makefile.m32` to:

- Move objects into a subdirectory.
- Add support for MS-DOS. Tested with DJGPP.
- Add support for Watt-32 (on MS-DOS).
- Add support for AmigaOS.
- Rename `Makefile.m32` to `Makefile.mk`
- Replace `ARCH` with `TRIPLET`.
- Build `tool_hugehelp.c` proper (when tools are available).
- Drop MS-DOS compatibility macro `USE_ZLIB` (replaced by `HAVE_LIBZ`)
- Add support for `ZLIB_LIBS` to override `-lz`.
- Omit object files when building examples.
- Default `CC` to `gcc` once again, for convenience. (Caveat: compiler
  name `cc` cannot be set now.)
- Set `-DCURL_NO_OLDIES` for examples, like autotools does.
- Delete `makefile.dj` files. Notice the configuration details and
  defaults are not retained with the new method.
- Delete `makefile.amiga` files. A successful build needs a few custom
  options. We're also not retaining all build details from the existing
  Amiga make files.
- Rename `Makefile.m32` to `Makefile.mk` to reflect that they are not
  Windows/MinGW32-specific anymore.
- Add support for new `CFG` options: `-map`, `-debug`, `-trackmem`
- Set `-DNDEBUG` by default.
- Allow using `-DOS=...` in all `lib/config-*.h` headers, syncing this
  with `config-win32.h`.
- Look for zlib parts in `ZLIB_PATH/include` and `ZLIB_PATH/lib`
  instead of bare `ZLIB_PATH`.

Note that existing build configurations for MS-DOS and AmigaOS likely
become incompatible with this change.

Example AmigaOS configuration:
```
export CROSSPREFIX=/opt/amiga/bin/m68k-amigaos-
export CC=gcc
export CPPFLAGS='-DHAVE_PROTO_BSDSOCKET_H'
export CFLAGS='-mcrt=clib2'
export LDFLAGS="${CFLAGS}"
export LIBS='-lnet -lm'
make -C lib -f Makefile.mk
make -C src -f Makefile.mk
```

Example MS-DOS configuration:
```
export CROSSPREFIX=/opt/djgpp/bin/i586-pc-msdosdjgpp-
export WATT_PATH=/opt/djgpp/net/watt
export ZLIB_PATH=/opt/djgpp
export OPENSSL_PATH=/opt/djgpp
export OPENSSL_LIBS='-lssl -lcrypt'
export CFG=-zlib-ssl
make -C lib -f Makefile.mk
make -C src -f Makefile.mk
```

Closes #9764

docs/examples/Makefile.am

@@ -24,8 +24,8 @@
 
 AUTOMAKE_OPTIONS = foreign nostdinc
 
-EXTRA_DIST = README.md Makefile.example Makefile.inc Makefile.m32 \
-  makefile.dj $(COMPLICATED_EXAMPLES) .checksrc
+EXTRA_DIST = README.md Makefile.example Makefile.inc Makefile.mk \
+  $(COMPLICATED_EXAMPLES) .checksrc
 
 # Specify our include paths here, and do it relative to $(top_srcdir) and
 # $(top_builddir), to ensure that these paths which belong to the library

docs/examples/Makefile.mk

@@ -22,37 +22,53 @@
 #
 #***************************************************************************
 
-# Build libcurl via lib/Makefile.m32 first.
+# Build libcurl via lib/Makefile.mk first.
 
 PROOT := ../..
 
-LDFLAGS  += -L$(PROOT)/lib
-LIBS     += -lcurl
-
 ifeq ($(findstring -static,$(CFG)),)
-  curl_DEPENDENCIES += $(PROOT)/lib/libcurl.dll.a
   DYN := 1
-else
-  curl_DEPENDENCIES := $(PROOT)/lib/libcurl.a
-  CPPFLAGS += -DCURL_STATICLIB
-  LDFLAGS += -static
 endif
 
-LIBS += -lws2_32
+### Common
+
+include $(PROOT)/lib/Makefile.mk
+
+### Local
+
+CPPFLAGS += -DCURL_NO_OLDIES
+LDFLAGS  += -L$(PROOT)/lib
+LIBS     := -lcurl $(LIBS)
+
+ifdef DYN
+  curl_DEPENDENCIES += $(PROOT)/lib/libcurl.dll.a
+else
+  curl_DEPENDENCIES := $(PROOT)/lib/libcurl.a
+  ifdef WIN32
+    CPPFLAGS += -DCURL_STATICLIB
+    LDFLAGS += -static
+  endif
+endif
+
+ifdef WIN32
+  LIBS += -lws2_32
+endif
 
 ### Sources and targets
 
 # Provides check_PROGRAMS
 include Makefile.inc
 
-TARGETS := $(patsubst %,%.exe,$(strip $(check_PROGRAMS) synctime))
-TOCLEAN := $(TARGETS:.exe=.o)
+ifdef WIN32
+check_PROGRAMS += synctime
+endif
 
-### Local rules
+TARGETS := $(patsubst %,%$(BIN_EXT),$(strip $(check_PROGRAMS)))
+TOCLEAN := $(TARGETS)
 
-%.exe: %.o $(curl_DEPENDENCIES)
-	$(CC) $(LDFLAGS) $(CURL_LDFLAGS_BIN) -o $@ $< $(LIBS)
+### Rules
 
-### Global script
+%$(BIN_EXT): %.c $(curl_DEPENDENCIES)
+	$(CC) $(CFLAGS) $(CPPFLAGS) $(LDFLAGS) $(CURL_LDFLAGS_BIN) $< -o $@ $(LIBS)
 
-include $(PROOT)/lib/Makefile.m32
+all: $(TARGETS)

docs/examples/makefile.dj

@@ -1,57 +0,0 @@
-#***************************************************************************
-#                                  _   _ ____  _
-#  Project                     ___| | | |  _ \| |
-#                             / __| | | | |_) | |
-#                            | (__| |_| |  _ <| |___
-#                             \___|\___/|_| \_\_____|
-#
-# Copyright (C) 1998 - 2022, Daniel Stenberg, <daniel@haxx.se>, et al.
-#
-# This software is licensed as described in the file COPYING, which
-# you should have received as part of this distribution. The terms
-# are also available at https://curl.se/docs/copyright.html.
-#
-# You may opt to use, copy, modify, merge, publish, distribute and/or sell
-# copies of the Software, and permit persons to whom the Software is
-# furnished to do so, under the terms of the COPYING file.
-#
-# This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
-# KIND, either express or implied.
-#
-# SPDX-License-Identifier: curl
-#
-###########################################################################
-
-TOPDIR = ../..
-
-include $(TOPDIR)/packages/DOS/common.dj
-
-CFLAGS += -DFALSE=0 -DTRUE=1
-
-LIBS = $(TOPDIR)/lib/libcurl.a
-
-ifeq ($(USE_SSL),1)
-  LIBS += $(OPENSSL_ROOT)/lib/libssl.a $(OPENSSL_ROOT)/lib/libcrypt.a
-endif
-
-ifeq ($(USE_IDNA),1)
-  LIBS += $(LIBIDN_ROOT)/lib/dj_obj/libidn.a -liconv
-endif
-
-LIBS += $(WATT32_ROOT)/lib/libwatt.a $(ZLIB_ROOT)/libz.a
-
-include Makefile.inc
-
-PROGRAMS = $(patsubst %,%.exe,$(check_PROGRAMS))
-
-all: $(PROGRAMS)
-	@echo Welcome to libcurl example program
-
-%.exe: %.c
-	$(CC) $(CFLAGS) -o $@ $^ $(LIBS)
-	@echo
-
-clean vclean realclean:
-	- rm -f $(PROGRAMS) depend.dj
-
--include depend.dj