GHA: enable -Wunused-macros in clang-tidy jobs

Also fix fallouts found.

Windows clang-tidy CI job is a little pickier than I'd prefer due to the
`_CURL_TESTS_CONCAT=ON` option used there, and all macros considered
local, thus checked by the compiler. Upside: it revealed macro usage
dynamics in tests. If too annoying, `first.h` may be opted-out from the
concat logic. Some macros may also be deleted instead of `#if 0`-ing.

Follow-up to e0e56e9ae4 #21550
Follow-up to 5fa5cb3825 #20593

Closes #21554

.github/workflows/linux.yml

@@ -321,6 +321,7 @@ jobs:
             install_steps: skiprun mbedtls-latest-intel rustls wolfssl-opensslextra-intel
             install_steps_brew: openssl@4 gsasl
             CC: clang-20
+            CFLAGS: -Wunused-macros
             LDFLAGS: >-
               -Wl,-rpath,/home/runner/wolfssl-opensslextra/lib
               -Wl,-rpath,/home/runner/mbedtls/lib
@@ -345,6 +346,7 @@ jobs:
             install_steps: skiprun
             install_steps_brew: libngtcp2 libnghttp3 c-ares
             CC: clang-20
+            CFLAGS: -Wunused-macros
             LDFLAGS: >-
               -Wl,-rpath,/home/linuxbrew/.linuxbrew/opt/openssl/lib
               -Wl,-rpath,/home/linuxbrew/.linuxbrew/opt/libngtcp2/lib

.github/workflows/macos.yml

@@ -300,6 +300,7 @@ jobs:
             compiler: clang
             install: llvm gnutls nettle libressl krb5 mbedtls gsasl rustls-ffi libssh fish
             install_steps: skiprun
+            CFLAGS: -Wunused-macros
             chkprefill: _chkprefill
             generate: >-
               -DCURL_USE_OPENSSL=ON -DOPENSSL_ROOT_DIR=/opt/homebrew/opt/libressl -DCURL_DEFAULT_SSL_BACKEND=openssl
@@ -316,6 +317,7 @@ jobs:
             compiler: clang
             install: llvm libnghttp3 libngtcp2 openldap krb5
             install_steps: skipall
+            CFLAGS: -Wunused-macros
             generate: >-
               -DCURL_USE_OPENSSL=ON -DOPENSSL_ROOT_DIR=/opt/homebrew/opt/openssl -DUSE_NGTCP2=ON
               -DLDAP_INCLUDE_DIR=/opt/homebrew/opt/openldap/include
@@ -445,6 +447,7 @@ jobs:
 
       - name: 'configure'
         env:
+          CFLAGS: '${{ matrix.build.CFLAGS }}'
           MATRIX_CHKPREFILL: '${{ matrix.build.chkprefill }}'
           MATRIX_CONFIGURE: '${{ matrix.build.configure }}'
           MATRIX_GENERATE: '${{ matrix.build.generate }}'
@@ -479,7 +482,6 @@ jobs:
               false
             fi
           else
-            export CFLAGS
             if [[ "${MATRIX_COMPILER}" = 'llvm'* ]]; then
               options+=" --target=$(uname -m)-apple-darwin"
             fi

.github/workflows/windows.yml

@@ -782,7 +782,7 @@ jobs:
         include:
           - { build: 'autotools', compiler: 'gcc' }
           - { build: 'cmake'    , compiler: 'gcc' }
-          - { build: 'cmake'    , compiler: 'clang-tidy', install_packages: 'clang-20 clang-tidy-20' }
+          - { build: 'cmake'    , compiler: 'clang-tidy', install_packages: 'clang-20 clang-tidy-20', CFLAGS: '-Wunused-macros' }
     steps:
       - name: 'install packages'
         timeout-minutes: 2
@@ -801,6 +801,8 @@ jobs:
         run: autoreconf -fi
 
       - name: 'configure'
+        env:
+          CFLAGS: '${{ matrix.CFLAGS }}'
         run: |
           if [ "${MATRIX_BUILD}" = 'cmake' ]; then
             if [ "${MATRIX_COMPILER}" = 'clang-tidy' ]; then