Fix and improve GitHub Actions CI

* Fixed problem with older compilers by explicitly installing the
  right compiler version.
* Split apart simple builds that can be well described by a build
  matrix, and builds that need special CMake defines or similar
  special snowflake handling.
* Ported some extras + examples builds from TravisCI to GitHub
  Actions.

.github/workflows/linux-simple-builds.yml

@@ -0,0 +1,91 @@
+name: Linux builds (basic)
+
+on: [push, pull_request]
+
+jobs:
+  build:
+    name: ${{matrix.cxx}}, C++${{matrix.std}}, ${{matrix.build_type}}
+    runs-on: ubuntu-20.04
+    strategy:
+      matrix:
+        cxx:
+#          - g++-6
+          - g++-7
+          - g++-8
+          - g++-9
+          - g++-10
+          - clang++-6.0
+          - clang++-7
+          - clang++-8
+          - clang++-9
+          - clang++-10
+        build_type: [Debug, Release]
+        std: [14]
+        include:
+        # cannot be installed on ubuntu-20.04 be default?
+#          - cxx: g++-6
+#            other_pkgs: g++-6
+          - cxx: g++-7
+            other_pkgs: g++-7
+          - cxx: g++-8
+            other_pkgs: g++-8
+          - cxx: clang++-6.0
+            other_pkgs: clang-6.0
+          - cxx: clang++-7
+            other_pkgs: clang-7
+          - cxx: clang++-8
+            other_pkgs: clang-8
+          - cxx: clang++-10
+            other_pkgs: clang-10
+          # Clang 6 + C++17
+          # does not work with the default libstdc++ version thanks
+          # to a disagreement on variant implementation.
+          # - cxx: clang++-6.0
+          #   build_type: Debug
+          #   std: 17
+          #   other_pkgs: clang-6.0
+          # - cxx: clang++-6.0
+          #   build_type: Release
+          #   std: 17
+          #   other_pkgs: clang-6.0
+          # Clang 10 + C++17
+          - cxx: clang++-10
+            build_type: Debug
+            std: 17
+            other_pkgs: clang-10
+          - cxx: clang++-10
+            build_type: Release
+            std: 17
+            other_pkgs: clang-10
+
+    steps:
+    - uses: actions/checkout@v2
+
+    - name: Prepare environment
+      run: sudo apt-get install -y ninja-build ${{matrix.other_pkgs}}
+
+    - 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}} \
+              -DCMAKE_CXX_EXTENSIONS=OFF \
+              -DCATCH_DEVELOPMENT_BUILD=ON \
+              -G Ninja
+
+    - name: Build tests + lib
+      working-directory: ${{runner.workspace}}/build
+      run: ninja
+
+    - 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