Add gcc 11 to CI

Author: andreasbuhr

.github/workflows/cmake.yml

@@ -31,6 +31,18 @@ jobs:
             cxx: "g++-10",
             cxxver: 20,
           }
+          - {
+            name: "Linux g++ 11 C++17",
+            os: ubuntu-20.04,
+            cxx: "g++-11",
+            cxxver: 17,
+          }
+          - {
+            name: "Linux g++ 11 C++20",
+            os: ubuntu-20.04,
+            cxx: "g++-11",
+            cxxver: 20,
+          }
           - {
             name: "Linux clang-10 C++17",
             os: ubuntu-20.04,
@@ -207,6 +219,15 @@ jobs:
         sudo add-apt-repository ppa:ubuntu-toolchain-r/ppa
         sudo apt-get install g++-10
 
+    - name: Install g++ 11
+      id: install_gcc_11
+      if: startsWith(matrix.config.os, 'ubuntu') && ( matrix.config.cxx == 'g++-11' )
+      shell: bash
+      working-directory: ${{ env.HOME }}
+      run: |
+        sudo add-apt-repository ppa:ubuntu-toolchain-r/ppa
+        sudo apt-get install g++-11
+
     - name: Create Build Environment
       # Some projects don't allow in-source building, so create a separate build directory
       # We'll use this as our working directory for all subsequent commands