Makefile.mk: drop in favour of autotools and cmake (MS-DOS, AmigaOS3)

Makefile.mk: drop in favour of autotools and cmake (MS-DOS, AmigaOS3)

`Makefile.mk` supported MS-DOS and Amiga, but `./configure` also
supported them in a better tested and more flexible way.

This patch also adds CMake support for MS-DOS/DJGPP and Amiga OS 3.

`Makefile.mk` was not maintained. Delete it in favour of first-tier
build methods.

Also include some non-MS-DOS/AmigaOS-specific tidy-up, see details at
the end of this message.

Details:

- fix/silence all MS-DOS/DJGPP build warnings and issues.
- add MS-DOS support to cmake.
  - default to `ENABLE_THREADED_RESOLVER=OFF` for MS-DOS.
  - add support for `WATT_ROOT`.
  - use static libcurl with MS-DOS.
  - fixup default CMake suffixes/prefixes for DJGPP.
  - disable hidden symbols for MS-DOS. Not supported on MS-DOS.
  - opt-in MS-DOS into `USE_UNIX_SOCKETS`.
- improve MS-DOS support in autotools.
  - default to `--disable-threaded-resolver` for MS-DOS.
- make sure to use `close_s()` (from Watt-32) with autotools and cmake.
  `Makefile.mk` used it before this patch.
- GHA: add DJGPP cmake (~30s) and autotools (~60s) build jobs.
  Also build tests and examples with cmake.
- improve AmigaOS support in autotools:
  - configure: detect `CloseSocket()` when it's a macro.
  - configure: fix `IoctlSocket` detection on AmigaOS.
  - curl-amissl.m4: pass AmiSSL libs to tests/servers.
- add AmigaOS3 support to cmake:
  - cmake: fix `HAVE_IOCTLSOCKET_CAMEL` and
    `HAVE_IOCTLSOCKET_CAMEL_FIONBIO` detections.
  - set necessary system libs.
  - add AmiSSL support.
  - inet_ntop, inet_pton: fix using it for AmigaOS. cmake detects them,
    and they did not compile with AmigaOS.
  - cmake: better sync `gethostname` detection with autotools.
    Fixes detection for AmigaOS, where `gethostname` is a macro.
  - cmake: fix `sys/utime.h` detection on AmigaOS.
  - cmake: force-disable `getaddrinfo` for AmigaOS.
  - cmake: tweak threading and static/shared default for AmigaOS.
  - cmake: rely on manual variable `AMIGA` to enable the platform.
- GHA: add AmigaOS cmake and autotools (~45s) jobs.
  Also build tests and examples with cmake.
- INSTALL: update MS-DOS and AmigaOS build instructions.
- amigaos: fix `-Wpointer-sign` and
  `zero or negative size array '_args'` in `Printf()`.
- amigaos: fix `-Wpointer-sign`
- amigaos: fix `-Wredundant-decls` `errno` and `h_errno`.
- amigaos: brute-force silence `lseek()` size warnings.
- amigaos: server/resolve: silence `-Wdiscarded-qualifiers`.
- amigaos: server/resolve: fix `-Wpointer-sign`.
- amigaos: fix `CURL_SA_FAMILY_T` type.
- nonblock: prefer `HAVE_IOCTLSOCKET_CAMEL_FIONBIO` for AmigaOS.
  `ioctl` is also detected, but fails when used. Make the above override
  it for a successful build.
  Authored-by: Darren Banfi
  Fixes #15537
  Closes #15603
- tftpd: prefer `HAVE_IOCTLSOCKET_CAMEL_FIONBIO` for AmigaOS.
- tftpd: tidy-up conditional code.
- curl: set stack size to 16384 for AmigaOS3/4
  Overriding the default 4096.
  Suggested-by: Darren Banfi
  Ref: https://github.com/curl/curl/pull/15543#issuecomment-2498783123
  Ref: https://wiki.amigaos.net/wiki/Controlling_Application_Stack
- functypes.h: fix `SEND_QUAL_ARG2` for AmigaOS.
- tftp: add missing cast in sendto() call for AmigaOS.
- getinfo: fix warning with AmigaOS.
- tool_operate: silence warning with AmigaOS
- amigaos: fix building libtests due to missing `RLIMIT_NOFILE`.
- curl_gethostname: silence warning for AmigaOS.
- ftp: silence `-Wtype-limits` for AmigaOS.
- libtest: fix timeval initialization for AmigaOS.
- examples: fix `timeval` initialization for AmigaOS.
- examples: silence warning for AmigaOS.
- configure: fix IPv6 detection for cross-builds.
- netrc: fix to build with AmigaOS cleanly.
- buildinfo: detect and add `DOS` tag for MS-DOS builds.
- buildinfo: add `AMIGA` to buildinfo.txt in auttools.
- build: move `USE_WATT32` macro definition to cmake/configure.

Non-MS-DOS/AmigeOS-specific tidy-ups:

- configure: sync `sa_family_t` detection with cmake.
- configure: sync `ADDRESS_FAMILY` detection signals with cmake.
- doh: use `CURL_SA_FAMILY_T`.
- lib: drop mingw-specific `CURL_SA_FAMILY_T` workaround.
- cmake: extend instead of override check-specific
  configurations/requirements.
  This allows to honor global requirements added earlier.
  Necessary for AmigaOS for example.
- cmake: omit warning on disabled IPv6 for MS-DOS and AmigaOS.
  No IPv6 support on these platforms. Also sync with autotools.
- lib1960: use libcurl `inet_pton()` wrapper.
- cmake: detect LibreSSL (to match autotools).
- cmake: say the specific OpenSSL flavour detected.
- hostip: add missing `HAVE_SOCKADDR_IN6_SIN6_SCOPE_ID` guard.
- lib: simplify classic mac feature guards.

Follow-up to a8861b6ccdd7ca35b6115588a578e36d765c9e38 #9764

Closes #15543