build: address some -Weverything warnings, update picky warnings

[vszakats]

-Weverything is not enabled by curl, and not recommended by LLVM,
because it may enable experimental options, and will result in new
fallouts after toolchain upgrades. This patch aims to fix/silence as much
as possible as found with llvm/clang 21.1.0. It also permanently enables
warnings that were fixed in source and deemed manageable in the future.
-Wformat warnings are addressed separately via #18343.

Fix/silence warnings in the source:

• typecheck-gcc.h: fix -Wreserved-identifier.
• lib: silence -Wcast-function-type-strict.
  For llvm 16+ or Apple clang 16+.
• asyn-ares: limit HAPPY_EYEBALLS_DNS_TIMEOUT to old c-ares versions.
• curl_trc: fix -Wc++-hidden-decl.
• doh: fix -Wc++-keyword.
• ftp: fix -Wreserved-identifier.
• ldap: fix -Wreserved-identifier.
• mqtt: comment unused macro to avoid warning.
• multi_ev: drop unused macros to avoid warnings.
• setopt: fix useless break; after return;.
• gtls, mbedtls, rustls: silence -Wconditional-uninitialized.
• socks_sspi, schannel, x509asn1: fix -Wimplicit-int-enum-cast.
• x509asn1: fix -Wc++-keyword.
• openssl: scope OSSL_UI_METHOD_CAST to avoid unused macro warning.
• libssh2, wolfssl: drop unused macros.
• curl_ngtcp2, curl_quiche, httpsrr, urlapi: drop/limit unused macros.
• tool_getparam: fix useless break; after return; or break;.
  Not normally enabled because it doesn't work with unity.
  clang 17.0.2: No "unreachable code" warning if source file included in another source file llvm/llvm-project#71046
• tool_operate: fix -Wc++-keyword.
• curlinfo: fix a -Wunsafe-buffer-usage.
• tests: silence -Wformat-non-iso.
• lib557: fix -Wreserved-identifier.
• lib1565: silence -Wconditional-uninitialized.

Enable the above clang warnings permanently in picky mode:

• -Wc++-hidden-decl
• -Wc++-keyword (except for Windows, where it collides with wchar_t)
• -Wcast-function-type-strict
• -Wcast-function-type
• -Wconditional-uninitialized
• -Wformat-non-iso (except for clang-cl)
• -Wreserved-identifier
• -Wtentative-definition-compat

Silence problematic -Weverything warnings globally (in picky mode):

• -Wused-but-marked-unused (88000+ hits) and
  -Wdisabled-macro-expansion (2600+ hits).
  Triggered by typecheck-gcc.h when building with clang 14+.
  Maybe there exists a way to fix within that header?
  Ref: https://discourse.llvm.org/t/removing-wused-but-marked-unused/55310
• -Wunsafe-buffer-usage. clang 16+. 7000+ hits.
  May be useful in theory, but such high volume of hits makes it
  impractical to review and possibly address. Meant for C++.
  Ref: https://clang.llvm.org/docs/SafeBuffers.html
  Ref: https://stackoverflow.com/questions/77017567/how-to-fix-code-to-avoid-warning-wunsafe-buffer-usage
  Ref: https://discourse.llvm.org/t/rfc-c-buffer-hardening/65734
  Ref: [-Wunsafe-buffer-usage] Add user documentation. llvm/llvm-project#111624
• -Wimplicit-void-ptr-cast. clang 21+. 1700+ hits.
  C++ warning, deemed pure noise.
  Ref: -Wtentative-definition-compat warning in openldap.c with clang 21 #18470 (comment)
• -Wswitch-default (180+ hits), -Wswitch-enum (190+ hits),
  -Wcovered-switch-default (20+ hits).
  Next to impossible to fix cleanly, esp. when the covered case
  branches depend on compile-time options.
• -Wdocumentation-unknown-command (8+ hits).
  Triggered in a few sources. Seems arbitrary and bogus.
• -Wpadded (550+ hits).
• -Wc++-keyword on Windows, where it collides with wchar_t.
  (100+ hits)
  Ref: Incorrect warning about wchar_t in C llvm/llvm-project#155988
• -Wreserved-macro-identifier. clang 13+. 5+ hits.
  Sometimes it's necessary to set external macros that use
  the reserved namespace. E.g. _CRT_NONSTDC_NO_DEPRECATE,
  __ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__, __NO_NET_API,
  possibly _REENTRANT, and more.
  It's not worth trying to silence them individually.
• -Wnonportable-system-include-path with clang-cl.
  It'd be broken by doing what the warning suggests.
• -Wformat-non-iso for clang-cl.

CMake PICKY_COMPILER=ON (the default) or ./configure
--enable-warnings (not the default) is required to enable these
silencing rules.

Also:

• autotools, cmake: fix Apple clang and mainline llvm version translations.
  Ref: https://en.wikipedia.org/wiki/Xcode#Toolchain_versions
• autotools, cmake: enable -Warray-compare for clang 20+.
  Follow-up to 4b7accd build: enable gcc-12/13+, clang-10+ picky warnings #17196
• cmake: fix to enable -Wmissing-variable-declarations at an earlier
  clang version.
• cmake: update internal logic to handle warning options with + in
  them.
• cmake: fix internal logic to match the whole option when looking
  into CMAKE_C_FLAGS for custom-disabled warnings.

Follow-up to b85cb8c #18485

---

w/o sp https://github.com/curl/curl/pull/18477/files?w=1

• enable warnings that were silenced in-source via this patch
• fixup clang version guard for Apple clang
• apply cmake changes to autotools too
• rebase on spacecheck: warn for 3+ empty lines in a row, fix fallouts #18478
• consider disabling -Wcast-function-type-strict globally? → Leaving them there for "awareness". But, there seems to be no way to fix these in most cases, because the callback types are either part of the public curl APIs, or part of public external APIs. In url.c it may be possible to wrap fwrite/fread in internal functions to match the signature exactly, but it's even more noise (and runtime perf hit) with no benefit for current code.

[vszakats]

Standing at:

6782 -Wunsafe-buffer-usage
 985 -Wformat
  12 -Wc++-hidden-decl
   8 -Wformat-non-iso

[vszakats]

Down to 985 -Wformat.

They are addressed via pending #18343.

[vszakats]

Azure Ubuntu mirrors superslow again.

[vszakats]

Some interesting edge cases and weird fallouts after enabling -Weverything in CI (as a one-time test):

• clang-tidy trips on the -Wimplicit-void-ptr-cast warning (clang-diagnostic-implicit-void-ptr-cast),
  likely because the warning isn't disabled, because the build uses Apple clang which doesn't support this option yet,
  but the clang-tidy tool is picked from Homebrew llvm, which does. (Apple clang doesn't ship with clang-tidy.)
  https://github.com/curl/curl/actions/runs/17558369300/job/49868303278?pr=18477
• The Xcode generator seems to be adding an include directory which doesn't exists, tripping -Wmissing-include-dirs. Only seen with iOS.
  https://github.com/curl/curl/actions/runs/17558369300/job/49868302983?pr=18477
  It seems like a CMake issue.

  [...] -Wno-semicolon-before-method-body -I/Users/runner/work/curl/curl/bld/lib/Debug-iphoneos/include -I/Users/runner/work/curl/curl/include -I/Users/runner/work/curl/curl/bld/lib -isystem /Users/runner/libressl/include -I/Users/runner/work/curl/curl/bld/build/libcurl_shared.build/Debug-iphoneos/DerivedSources-normal/arm64 -I/Users/runner/work/curl/curl/bld/build/libcurl_shared.build/Debug-iphoneos/DerivedSources/arm64 -I/Users/runner/work/curl/curl/bld/build/libcurl_shared.build/Debug-iphoneos/DerivedSources -F/Users/runner/work/curl/curl/bld/lib/Debug-iphoneos -fPIC -fvisibility\=hidden [...]
  [...]
  clang: error: no such include directory: '/Users/runner/work/curl/curl/bld/lib/Debug-iphoneos/include' [-Werror,-Wmissing-include-dirs]
  clang: error: no such include directory: '/Users/runner/work/curl/curl/bld/build/libcurl_shared.build/Debug-iphoneos/DerivedSources-normal/arm64' [-Werror,-Wmissing-include-dirs]
  clang: error: no such include directory: '/Users/runner/work/curl/curl/bld/build/libcurl_shared.build/Debug-iphoneos/DerivedSources/arm64' [-Werror,-Wmissing-include-dirs]
  clang: error: no such include directory: '/Users/runner/work/curl/curl/bld/build/libcurl_shared.build/Debug-iphoneos/DerivedSources' [-Werror,-Wmissing-include-dirs]
  

• OpenBSD dies with LLVM ERROR: out of memory when compiling libtests.
  https://github.com/curl/curl/actions/runs/17558369306/job/49868302856?pr=18477#step:3:2015
• FreeBSD exhibits this warning:

  /home/runner/work/curl/curl/lib/mime.c:363:17: error: '_Generic' is incompatible with C standards before C11 [-Werror,-Wpre-c11-compat]
    363 |   base = strdup(basename(filename));
        |                 ^
  /usr/include/libgen.h:60:21: note: expanded from macro 'basename'
     60 | #define basename(x)     __generic(x, const char *, __old_basename, basename)(x)
        |                         ^
  /usr/include/sys/cdefs.h:314:2: note: expanded from macro '__generic'
    314 |         _Generic(expr, t: yes, default: no)
        |         ^
  1 error generated.
  

  https://github.com/curl/curl/actions/runs/17558369306/job/49868302863?pr=18477#step:3:1996