Makefile.m32: major rework [ci skip]

Makefile.m32: major rework [ci skip]

This patch overhauls `Makefile.m32` scripts, fixing a list of quirks,
making its behaviour and customization envvars align better with other
build systems, aiming for less code, that is easier to read, use and
maintain.

Details:
- Rename customization envvars:
  `CURL_CC` -> `CC`
  `CURL_RC` -> `RC`
  `CURL_AR` -> `AR`
  `CURL_LDFLAG_EXTRAS_DLL` -> `CURL_LDFLAGS_LIB`
  `CURL_LDFLAG_EXTRAS_EXE` -> `CURL_LDFLAGS_BIN`
- Drop `CURL_STRIP` and `CURL_RANLIB`. These tools are no longer used.
- Accept `CFLAGS`, `CPPFLAGS`, `RCFLAGS`, `LDFLAGS` and `LIBS` envvars.
- Drop `CURL_CFLAG_EXTRAS`, `CURL_LDFLAG_EXTRAS`, `CURL_RCFLAG_EXTRAS` in
  favor of the above.
- Do not automatically enable `zlib` with `libssh2`. `zlib` is optional
  with `libssh2`.
- Omit unnecessary `CPPFLAGS` options when building `curl.exe` and
  examples.
- Drop support for deprecated `-winssl` `CFG` option. Use `-schannel`
  instead.
- Avoid late evaluation where not necessary (`=` -> `:=`).
- Drop support for `CURL_DLL_A_SUFFIX` to override the implib suffix.
  Instead, use the standard naming scheme by default: `libcurl.dll.a`.
  The toolchain recognizes the name, and selects it automatically when
  asking for a `-shared` vs. `-static` build.
- Stop applying `strip` to `libcurl.a`. Follow-up from
  16a58e9f93c7e89e1f87720199388bcfcfa148a4. There was no debug info to
  strip since then.
- Stop setting `-O3`, `-W`, `-Wall` options. You can add these to
  `CFLAGS` as desired.
- Always enable `-DCURL_DISABLE_OPENSSL_AUTO_LOAD_CONFIG` with OpenSSL,
  to avoid that vulnerability on Windows.
- Add `-lbrotlicommon` to `LIBS` when using `brotli`.
- Do not enable `-nghttp3` without `-ngtcp2`.
- `-ssh2` and `-rtmp` options no longer try to auto-select a TLS-backend.
  You need to set the backend explicitly. This scales better and avoids
  issues with certain combinations (e.g. `libssh2` + `wolfssl` with no
  `schannel`).
- Default to OpenSSL TLS-backend with `ngtcp2`. Possible to override via
  `NGTCP2_LIBS`.
- Old, alternate method of enabling components (e.g. `SSH2=1`) no longer
  supported.
- Delete `SPNEGO` references. They were no-ops.
- Drop support for Win9x environments.
- Allow setting `OPENSSL_LIBS` independently from `OPENSSL_LIBPATH`.
- Support autotools/CMake `libssh2` builds by default.
- Respect `CURL_DLL_SUFFIX` in `-dyn` mode when building `curl.exe` and
  examples.
- Assume standard directory layout with `LIBCARES_PATH`. (Instead of the
  long gone embedded one.)
- Stop static linking with c-ares by default. Add
  `CPPFLAGS=-DCARES_STATICLIB` to enable it.
- Reorganize internal layout to avoid redundancy and emit clean diffs
  between src/lib and example make files.
- Delete unused variables.
- Code cleanups/rework.
- Comment and indentation fixes.

Closes #9632