:alembic: add C++20 build

.github/workflows/ubuntu.yml

diff --git a/.github/workflows/ubuntu.yml b/.github/workflows/ubuntu.yml
index 9792431..58fdbd8 100644
--- a/.github/workflows/ubuntu.yml
+++ b/.github/workflows/ubuntu.yml
@@ -42,3 +42,43 @@
       run: cmake --build build --parallel 10
     - name: test
       run: cd build ; ctest -j 10 --output-on-failure
+
+  gcc_build_cxx20:
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v1
+      - name: install_gcc
+        run: |
+          sudo apt update
+          sudo apt install gcc-10 g++-10
+        shell: bash
+      - name: cmake
+      run: cmake -S . -B build -D CMAKE_BUILD_TYPE=Debug -DJSON_BuildTests=On -DCMAKE_CXX_STANDARD=20 -DCMAKE_CXX_STANDARD_REQUIRED=ON
+        env:
+          CC:   gcc-10
+          CXX:  g++-10
+      - name: build
+        run: cmake --build build --parallel 10
+      - name: test
+        run: cd build ; ctest -j 10 --output-on-failure
+
+  clang_build_cxx20:
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v1
+    - name: install_gcc
+      run: |
+           sudo apt update
+           sudo apt install clang-10
+      shell: bash
+    - name: cmake
+      run: cmake -S . -B build -D CMAKE_BUILD_TYPE=Debug -DJSON_BuildTests=On -DCMAKE_CXX_STANDARD=20 -DCMAKE_CXX_STANDARD_REQUIRED=ON
+      env:
+        CC:   clang-10
+        CXX:  clang++-10
+    - name: build
+      run: cmake --build build --parallel 10
+    - name: test
+      run: cd build ; ctest -j 10 --output-on-failure