meson: g++-[789] build

Test them as they are still supported.

Signed-off-by: Rosen Penev <rosenp@gmail.com>

.github/workflows/on_PR_meson.yaml

@@ -8,21 +8,23 @@ concurrency:
 
 jobs:
   Ubuntu:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
-    name: Linux-${{matrix.deps}}
+    name: Linux-GCC${{matrix.cxx}}-${{matrix.deps}}
     strategy:
       matrix:
+        cxx: ['7', '8', '9']
         deps: ['forcefallback', 'default']
     steps:
       - uses: actions/checkout@v3
 
       - name: Install packages
         run: |
+          sudo apt install -y g++-${{matrix.cxx}}
           python3 -m pip install meson ninja
 
       - name: Sanity Checks
         run: |
-          meson setup "${{github.workspace}}/build" --wrap-mode=${{matrix.deps}}
+          env CXX=g++-${{matrix.cxx}} meson setup "${{github.workspace}}/build" --wrap-mode=${{matrix.deps}}
           meson compile -C "${{github.workspace}}/build"
 
   VisualStudio: