Third attempt to use environment variables to select the compiler.

mps/.github/workflows/build-and-test.yml

@@ -26,13 +26,17 @@ jobs:
         env:
           - CC: gcc
           - CC: clang
+      exclude:
+        - os: macos-latest
+          env:
+            - CC: gcc
 
     runs-on: ${{ matrix.os }}
 
     steps:
     - uses: actions/checkout@v3
     - name: configure
-      run: ./configure
+      run: CC=${{ matrix.env.cc }} ./configure
     - name: make
       run: make
     - name: make test