cmake: silence silly Apple clang warnings in C89 mode, test in CI

- `stdbool.h` is also included via system headers. Disabling it from curl
  does not fix it. Silencing lots of these:
  ```
  curl/lib/curlx/warnless.h:64:1: warning: '_Bool' is a C99 extension [-Wc99-extensions]
   64 | bool curlx_sztouz(ssize_t sznum, size_t *puznum);
      | ^
  /Library/Developer/CommandLineTools/usr/lib/clang/17/include/stdbool.h:24:14: note: expanded from macro 'bool'
   24 | #define bool _Bool
      |              ^
  ```

- silence `-Wcomma` warnings.
  in favor of the global silencing approach, since a couple of more of
  these were hit (in vquic, tool1622, unit1309, unit1636), and it seems
  silly to update them all.
  Revert e8189c4420 #20362

Also:
- cmake: include C standard in 'platform flags' log line.
- GHA/macos: switch a job to C89 to verify.
- GHA/linux: show 'C89' in job names.

Ref: https://cmake.org/cmake/help/v3.7/variable/CMAKE_C_STANDARD.html

Closes #20363

.github/workflows/linux.yml

@@ -185,12 +185,12 @@ jobs:
             tflags: '--min=910 951 to 9999'
             generate: -DENABLE_DEBUG=ON -DENABLE_THREADED_RESOLVER=OFF
 
-          - name: 'openssl intel'
+          - name: 'openssl intel C89'
             install_packages: libssh-dev
             install_steps: pytest
             configure: CFLAGS=-std=gnu89 --with-openssl --with-libssh --enable-debug
 
-          - name: 'openssl arm'
+          - name: 'openssl arm C89'
             image: ubuntu-24.04-arm
             install_steps: pytest
             configure: CFLAGS=-std=gnu89 --with-openssl --enable-debug --disable-verbose

.github/workflows/macos.yml

@@ -257,11 +257,11 @@ jobs:
             install: libressl
             install_steps: pytest
             generate: -DENABLE_DEBUG=ON -DOPENSSL_ROOT_DIR=/opt/homebrew/opt/libressl -DCURL_DISABLE_LDAP=ON -DCURL_BROTLI=OFF -DCURL_ZSTD=OFF -DCURL_USE_LIBSSH2=OFF
-          - name: 'OpenSSL 10.15'
+          - name: 'OpenSSL 10.15 C89'
             compiler: clang
             install: libnghttp3 libngtcp2
             install_steps: pytest
-            generate: -DENABLE_DEBUG=ON -DOPENSSL_ROOT_DIR=/opt/homebrew/opt/openssl -DUSE_NGTCP2=ON -DCURL_BROTLI=OFF -DCURL_ZSTD=OFF -DCURL_USE_LIBSSH2=OFF
+            generate: -DENABLE_DEBUG=ON -DOPENSSL_ROOT_DIR=/opt/homebrew/opt/openssl -DUSE_NGTCP2=ON -DCURL_BROTLI=OFF -DCURL_ZSTD=OFF -DCURL_USE_LIBSSH2=OFF -DCMAKE_C_STANDARD=90
             macos-version-min: '10.15'
           - name: 'OpenSSL SecTrust'
             compiler: clang