Move some Mac builds from Travis to GitHub Actions

.github/workflows/linux-builds.yml

@@ -7,7 +7,7 @@ jobs:
     runs-on: ubuntu-20.04
     strategy:
       matrix:
-        cxx: 
+        cxx:
 #          - g++-6
           - g++-7
           - g++-8

.github/workflows/mac-builds.yml

@@ -0,0 +1,39 @@
+name: Mac builds
+
+on: [push, pull_request]
+
+jobs:
+  build:
+    runs-on: macos-10.15
+    strategy:
+      matrix:
+        cxx:
+          - g++-9
+          - clang++
+        build_type: [Debug, Release]
+        std: [14]
+
+    steps:
+    - uses: actions/checkout@v2
+
+    - name: Configure build
+      working-directory: ${{runner.workspace}}
+      env:
+        CXX: ${{matrix.cxx}}
+        CXXFLAGS: ${{matrix.cxxflags}}
+      # Note: $GITHUB_WORKSPACE is distinct from ${{runner.workspace}}.
+      #       This is important
+      run: |
+        cmake -Bbuild -H$GITHUB_WORKSPACE -DCMAKE_BUILD_TYPE=${{matrix.build_type}} \
+              -DCMAKE_CXX_STANDARD=${{matrix.std}} -DCATCH_DEVELOPMENT_BUILD=ON
+
+    - name: Build tests + lib
+      working-directory: ${{runner.workspace}}/build
+      run: make -j 2
+
+    - name: Run tests
+      env:
+          CTEST_OUTPUT_ON_FAILURE: 1
+      working-directory: ${{runner.workspace}}/build
+      # Hardcode 2 cores we know are there
+      run: ctest -C ${{matrix.build_type}} -j 2