CI now tests against macos-latest too

Signed-off-by: Rainer Schoenberger <[email protected]>

.github/workflows/cmake.yml

@@ -16,7 +16,11 @@ 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: [ubuntu-latest, macos-latest]
 
     steps:
     - uses: actions/checkout@v2