Found them in autotools builds. They were missing from
CMake in curl/libssh2. One of them was also missing from
autotools llvm/clang.

Ref: curl/curl#12324
Ref: libssh2/libssh2#1219

Follow-up to 033f8e2 curl#12324

Follow-up to 033f8e2 curl#12324

Follow-up to 033f8e2 curl#12324

Follow-up to 033f8e2 curl#12324

Enable more picky compiler warnings. I've found these options in the
nghttp3 project when implementing the CMake quick picky warning
functionality for it [1].

`-Wunused-macros` was too noisy to keep around, but fixed a few issues
it revealed while testing.

- autotools: reflect the more precisely-versioned clang warnings.
  Follow-up to 033f8e2 #12324
- autotools: sync between clang and gcc the way we set `no-multichar`.
- autotools: avoid setting `-Wstrict-aliasing=3` twice.
- autotools: disable `-Wmissing-noreturn` for MSYS gcc targets [2].
  It triggers in libtool-generated stub code.

- lib/timeval: delete a redundant `!MSDOS` guard from a `WIN32` branch.

- lib/curl_setup.h: delete duplicate declaration for `fileno`.
  Added in initial commit ae1912c
  (1999-12-29). This suggests this may not be needed anymore, but if
  it does, we may restore this for those specific (non-Windows) systems.
- lib: delete unused macro `FTP_BUFFER_ALLOCSIZE` since
  c1d6fe2.
- lib: delete unused macro `isxdigit_ascii` since
  f65f750.
- lib/mqtt: delete unused macro `MQTT_HEADER_LEN`.
- lib/multi: delete unused macro `SH_READ`/`SH_WRITE`.
- lib/hostip: add `noreturn` function attribute via new `CURL_NORETURN`
  macro.
- lib/mprintf: delete duplicate declaration for `Curl_dyn_vprintf`.
- lib/rand: fix `-Wunreachable-code` and related fallouts [3].
- lib/setopt: fix `-Wunreachable-code-break`.
- lib/system_win32 and lib/timeval: fix double declarations for
  `Curl_freq` and `Curl_isVistaOrGreater` in CMake UNITY mode [4].
- lib/warnless: fix double declarations in CMake UNITY mode [5].
  This was due to force-disabling the header guard of `warnless.h` to
  to reapply it to source code coming after `warnless.c` in UNITY
  builds. This reapplied declarations too, causing the warnings.
  Solved by adding a header guard for the lines that actually need
  to be reapplied.
- lib/vauth/digest: fix `-Wunreachable-code-break` [6].
- lib/vssh/libssh2: fix `-Wunreachable-code-break` and delete redundant
  block.
- lib/vtls/sectransp: fix `-Wunreachable-code-break` [7].
- lib/vtls/sectransp: suppress `-Wunreachable-code`.
  Detected in `else` branches of dynamic feature checks, with results
  known at compile-time, e.g.
  ```c
  if(SecCertificateCopySubjectSummary)  /* -> true */
  ```
  Likely fixable as a separate micro-project, but given SecureTransport
  is deprecated anyway, let's just silence these locally.
- src/tool_help: delete duplicate declaration for `helptext`.
- src/tool_xattr: fix `-Wunreachable-code`.
- tests: delete duplicate declaration for `unitfail` [8].
- tests: delete duplicate declaration for `strncasecompare`.
- tests/libtest: delete duplicate declaration for `gethostname`.
  Originally added in 687df5c
  (2010-08-02).
  Got complicated later: c49e968
  If there are still systems around with warnings, we may restore the
  prototype, but limited for those systems.
- tests/lib2305: delete duplicate declaration for
  `libtest_debug_config`.
- tests/h2-download: fix `-Wunreachable-code-break`.

[1] https://github.com/ngtcp2/nghttp3/blob/a70edb08e954d690e8fb2c1df999b5a056f8bf9f/cmake/PickyWarningsC.cmake
[2] https://ci.appveyor.com/project/curlorg/curl/builds/48553586/job/3qkgjauiqla5fj45?fullLog=true#L1675
[3] https://github.com/curl/curl/actions/runs/6880886309/job/18716044703?pr=12331#step:7:72
    https://github.com/curl/curl/actions/runs/6883016087/job/18722707368?pr=12331#step:7:109
[4] https://ci.appveyor.com/project/curlorg/curl/builds/48555101/job/9g15qkrriklpf1ut#L204
[5] https://ci.appveyor.com/project/curlorg/curl/builds/48555101/job/9g15qkrriklpf1ut#L218
[6] https://github.com/curl/curl/actions/runs/6880886309/job/18716042927?pr=12331#step:7:290
[7] https://github.com/curl/curl/actions/runs/6891484996/job/18746659406?pr=12331#step:9:1193
[8] https://github.com/curl/curl/actions/runs/6882803986/job/18722082562?pr=12331#step:33:1870

Closes #12331