GHA/macos: drop compiler matrix for macos jobs

To reduce the number of jobs, and the time spent running macos runners.
The curl codebase has just a few Apple-specific parts, and they remain
well-covered after this patch.

Makes the number of jobs 24, down from 32.

Perhaps more jobs could be merged or dropped, and CPU use reduced by
moving some to cmake. Unique, but not Apple-specific jobs may also be
moved to Linux.

Follow-up to b869932392 #20178

Closes #20187

.github/workflows/macos.yml

@@ -205,21 +205,20 @@ jobs:
           fi
 
   macos:
-    name: "${{ matrix.build.generate && 'CM' || 'AM' }} ${{ matrix.compiler }} ${{ matrix.build.name }}"
+    name: "${{ matrix.build.generate && 'CM' || 'AM' }} ${{ matrix.build.compiler }} ${{ matrix.build.name }}"
     runs-on: 'macos-15'
     timeout-minutes: 25
     env:
       DEVELOPER_DIR: "/Applications/Xcode${{ matrix.build.xcode && format('_{0}', matrix.build.xcode) || '' }}.app/Contents/Developer"
-      CC: '${{ matrix.compiler }}'
+      CC: '${{ matrix.build.compiler }}'
       MATRIX_BUILD: ${{ matrix.build.generate && 'cmake' || 'autotools' }}
-      MATRIX_COMPILER: '${{ matrix.compiler }}'
+      MATRIX_COMPILER: '${{ matrix.build.compiler }}'
       MATRIX_INSTALL: '${{ matrix.build.install }}'
       MATRIX_INSTALL_STEPS: '${{ matrix.build.install_steps }}'
       MATRIX_MACOS_VERSION_MIN: '${{ matrix.build.macos-version-min }}'
     strategy:
       fail-fast: false
       matrix:
-        compiler: [clang, llvm@18, gcc-13]
         build:
           # autotools
           - name: '!ssl !debug brotli zstd'
@@ -287,6 +286,7 @@ jobs:
             tflags: '--test-event --min=1300'
           # cmake
           - name: 'OpenSSL gsasl rtmp AppleIDN SecTrust +examples'
+            compiler: clang
             install: libnghttp3 libngtcp2 gsasl rtmpdump
             generate: -DOPENSSL_ROOT_DIR=/opt/homebrew/opt/openssl -DCURL_USE_GSASL=ON -DUSE_LIBRTMP=ON -DUSE_APPLE_IDN=ON -DUSE_NGTCP2=ON -DCURL_DISABLE_VERBOSE_STRINGS=ON -DUSE_APPLE_SECTRUST=ON
           - name: 'MultiSSL AppleIDN clang-tidy +examples'
@@ -313,9 +313,11 @@ jobs:
               -DCURL_CLANG_TIDY=ON -DCLANG_TIDY=/opt/homebrew/opt/llvm/bin/clang-tidy
 
           - name: 'LibreSSL openldap krb5 c-ares +examples'
+            compiler: clang
             install: libressl krb5 openldap
             generate: -DENABLE_DEBUG=ON -DOPENSSL_ROOT_DIR=/opt/homebrew/opt/libressl -DENABLE_ARES=ON -DCURL_USE_GSSAPI=ON -DGSS_ROOT_DIR=/opt/homebrew/opt/krb5 -DLDAP_INCLUDE_DIR=/opt/homebrew/opt/openldap/include -DLDAP_LIBRARY=/opt/homebrew/opt/openldap/lib/libldap.dylib -DLDAP_LBER_LIBRARY=/opt/homebrew/opt/openldap/lib/liblber.dylib
           - name: 'wolfSSL !ldap brotli zstd'
+            compiler: clang
             install: brotli wolfssl zstd
             install_steps: pytest
             generate: -DCURL_USE_WOLFSSL=ON -DCURL_DISABLE_LDAP=ON -DUSE_ECH=ON
@@ -325,6 +327,7 @@ jobs:
             install_steps: codeset-test
             generate: -DCURL_USE_MBEDTLS=ON -DCURL_DISABLE_LDAP=ON -DCURL_DEFAULT_SSL_BACKEND=mbedtls -DCURL_USE_OPENSSL=ON -DUSE_APPLE_IDN=ON
           - name: 'GnuTLS !ldap krb5 +examples'
+            compiler: clang
             install: gnutls nettle krb5
             generate: -DENABLE_DEBUG=ON -DCURL_USE_GNUTLS=ON -DCURL_USE_OPENSSL=OFF -DCURL_USE_GSSAPI=ON -DGSS_ROOT_DIR=/opt/homebrew/opt/krb5 -DCURL_DISABLE_LDAP=ON -DUSE_SSLS_EXPORT=ON
           - name: 'aws-lc'
@@ -354,14 +357,6 @@ jobs:
             install_steps: torture
             generate: -DENABLE_DEBUG=ON -DBUILD_SHARED_LIBS=OFF -DENABLE_THREADED_RESOLVER=OFF -DOPENSSL_ROOT_DIR=/opt/homebrew/opt/openssl -DUSE_OPENSSL_QUIC=ON
             tflags: '-t --shallow=25 --min=628 1251 to 9999'
-        exclude:
-          # opt out jobs from combinations that have the compiler set manually
-          - { compiler: llvm@18, build: { compiler: 'clang' } }
-          - { compiler: llvm@18, build: { compiler: 'gcc-13' } }
-          - { compiler: gcc-13, build: { compiler: 'clang' } }
-          - { compiler: gcc-13, build: { compiler: 'llvm@18' } }
-          - { compiler: clang, build: { compiler: 'gcc-13' } }
-          - { compiler: clang, build: { compiler: 'llvm@18' } }
 
     steps:
       - name: 'brew install'