Maybe this will work?

.github/workflows/cmake.yml

@@ -12,7 +12,11 @@ jobs:
     # well on Windows or Mac.  You can convert this to a matrix build if you need
     # cross-platform coverage.
     # See: https://docs.github.com/en/free-pro-team@latest/actions/learn-github-actions/managing-complex-workflows#using-a-build-matrix
-    runs-on: ubuntu-latest
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        os: [macos-latest, ubuntu-18.04, windows-latest]
+        compiler: [gcc-8, clang]
 
     steps:
     - uses: actions/checkout@v2
@@ -39,6 +43,8 @@ jobs:
     - name: Build
       working-directory: ${{github.workspace}}/build
       shell: bash
+      env:
+        CC: ${{ matrix.compiler }}
       # Execute the build.  You can specify a specific target with "--target <NAME>"
       run: cmake --build . --config $BUILD_TYPE