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

1 files changed, 2 insertions, 0 deletions

diff --git a/lib/config-amigaos.h b/lib/config-amigaos.h
index b7356c762..20ea27df7 100644
--- a/lib/config-amigaos.h
+++ b/lib/config-amigaos.h
@@ -78,7 +78,9 @@
 #define USE_MANUAL 1
 #define CURL_DISABLE_LDAP 1
 
+#ifndef OS
 #define OS "AmigaOS"
+#endif
 
 #define PACKAGE "curl"
 #define PACKAGE_BUGREPORT "a suitable mailing list: https://curl.se/mail/"