clang-tidy: add to CI, add cmake support, fix fallouts

build:
- autotools: fix to build generated sources for the `tidy` target.
- autotools: allow passing custom clang-tidy options via
  `CURL_CLANG_TIDYFLAGS` env.
- cmake: add `CURL_CLANG_TIDY` option to configure for `clang-tidy`.
  Also add:
  - `CLANG_TIDY` variable to customize the `clang-tidy` tool.
  - `CURL_CLANG_TIDYFLAGS` to pass custom options to `clang-tidy`.
- apply `--enable-werror` and `-DCURL_WERROR=ON` to `clang-tidy`.

CI/GHA:
- add clang-tidy job for Linux, using autotools and clang-tidy v18.
  This one needs to disable `clang-analyzer-valist.Uninitialized`
  to avoid false positives:
  https://github.com/llvm/llvm-project/issues/40656
  Duration: 5.5 minutes
- add clang-tidy job for macOS, using cmake and clang-tidy v19.
  This one also covers tests and examples, and doesn't hit the false
  positives seen with llvm v18 and earlier.
  Duration: 4.5 minutes
- Linux/macOS: skip installing test dependencies when not building or
  running tests.

fix fallouts reported by `clang-tidy`:
- lib:
  - cf-h2-proxy: unused assignment in non-debug builds.
  - cf-socket: silence warning.
    FIXME: https://github.com/curl/curl/pull/15825#issuecomment-2561867769
  - ftp: NULL passed to `strncmp()`.
  - http2: NULL-ptr deref.
  - mprintf: silence warning.
- src/tool_writeout: NULL passed to `fputs()`.
- examples:
  - invalid file pointers.
  - missing `fclose()`.
- tests:
  - http/clients/hx-download: memory leaks on error.
  - http/clients/hx-download: memory leak on repeat `-r` option.
  - server: double `fclose()`.
    https://www.man7.org/linux/man-pages/man3/fclose.3.html
  - server: invalid file pointer/handle.
  - server/getpart: unused assignments.
  - server/mqttd: leak on failed `realloc()`.
  - server/tftpd: NULL passed to `strcmp()`.

Closes #15825

.github/workflows/macos.yml

@@ -131,6 +131,10 @@ jobs:
           - name: 'OpenSSL gsasl rtmp AppleIDN'
             install: gsasl rtmpdump
             generate: -DOPENSSL_ROOT_DIR=$(brew --prefix openssl) -DCURL_USE_GSASL=ON -DUSE_LIBRTMP=ON -DUSE_APPLE_IDN=ON
+          - name: 'OpenSSL AppleIDN clang-tidy +examples'
+            install: llvm
+            generate: -DOPENSSL_ROOT_DIR=$(brew --prefix openssl) -DUSE_APPLE_IDN=ON -DCURL_CLANG_TIDY=ON -DCLANG_TIDY=$(brew --prefix llvm)/bin/clang-tidy
+            clang-tidy: true
           - name: 'OpenSSL +static libssh +examples'
             install: libssh
             generate: -DOPENSSL_ROOT_DIR=$(brew --prefix openssl) -DBUILD_STATIC_LIBS=ON -DCURL_USE_LIBSSH2=OFF -DCURL_USE_LIBSSH=ON
@@ -161,6 +165,8 @@ jobs:
           - { compiler: llvm@15, build: { macos-version-min: '10.15' } }
           - { compiler: llvm@15, build: { torture: true } }
           - { compiler: gcc-12, build: { torture: true } }
+          - { compiler: llvm@15, build: { clang-tidy: true } }
+          - { compiler: gcc-12, build: { clang-tidy: true } }
           # opt out jobs from combinations that have the compiler set manually
           - { compiler: llvm@15, build: { compiler: 'clang' } }
           - { compiler: llvm@15, build: { compiler: 'gcc-12' } }
@@ -176,7 +182,9 @@ jobs:
         # https://github.com/curl/curl/runs/4095721123?check_suite_focus=true
         run: |
           echo ${{ matrix.build.generate && 'ninja' || 'automake libtool' }} \
-            pkgconf libpsl libssh2 nghttp2 stunnel ${{ matrix.build.install }} | xargs -Ix -n1 echo brew '"x"' > /tmp/Brewfile
+            pkgconf libpsl libssh2 \
+            ${{ !matrix.build.clang-tidy && 'nghttp2 stunnel' || '' }} \
+            ${{ matrix.build.install }} | xargs -Ix -n1 echo brew '"x"' > /tmp/Brewfile
           while [[ $? == 0 ]]; do for i in 1 2 3; do brew update && brew bundle install --no-lock --file /tmp/Brewfile && break 2 || { echo Error: wait to try again; sleep 10; } done; false Too many retries; done
 
       - name: 'brew unlink openssl'
@@ -281,12 +289,14 @@ jobs:
           fi
 
       - name: 'install test prereqs'
+        if: ${{ !matrix.build.clang-tidy }}
         run: |
           python3 -m venv $HOME/venv
           source $HOME/venv/bin/activate
           python3 -m pip install -r tests/requirements.txt
 
       - name: 'run tests'
+        if: ${{ !matrix.build.clang-tidy }}
         timeout-minutes: ${{ matrix.build.torture && 20 || 10 }}
         run: |
           export TFLAGS='-j20 ${{ matrix.build.tflags }}'