CI: compile main app with clang and GCC for several versions

.github/actions/setup-compilers/action.yml

@@ -0,0 +1,30 @@
+name: 'Set CC and CXX env vars for the configured compiler'
+
+inputs:
+  compiler:
+    description: 'Use $suite-$version'
+    required: true
+
+runs:
+  using: 'composite'
+  steps:
+    - name: Set the env vars with some bash
+      shell: bash
+      run: |
+        set -ex
+        input=${{ inputs.compiler }}
+        split=(${input//-/ })
+        suite=${split[0]}
+        version=${split[1]}
+        case $suite in
+          gnu)
+            echo "CC=gcc-$version" >> $GITHUB_ENV
+            echo "CXX=g++-$version" >> $GITHUB_ENV
+            ;;
+          llvm)
+            echo "CC=clang-$version" >> $GITHUB_ENV
+            echo "CXX=clang++-$version" >> $GITHUB_ENV
+            echo "CXXFLAGS=-stdlib=libc++" >> $GITHUB_ENV
+            echo "LDFLAGS=-stdlib=libc++" >> $GITHUB_ENV
+            ;;
+        esac

.github/workflows/main-app.yml

@@ -24,9 +24,6 @@ env:
   hemelb_install_prefix: ${{github.workspace}}/install
   # VMs have 2 cores, double as compilation often IO bound
   CMAKE_BUILD_PARALLEL_LEVEL: 4
-  # Latest Gnu compiler on Ubuntu 22.04
-  CC: gcc-12
-  CXX: g++-12
 
 jobs:
   basic-checks:
@@ -45,10 +42,16 @@ jobs:
   fluid_only_build:
     name: Build in fluid-only mode
     runs-on: ubuntu-22.04
-
+    strategy:
+      matrix:
+        compiler: [gnu-11, gnu-12, gnu-13]
     steps:
     - uses: actions/checkout@v3
 
+    - uses: ./.github/actions/setup-compilers
+      with:
+        compiler: ${{ matrix.compiler }}
+
     - uses: ./.github/actions/install-hemelb-deps
       with:
         name: fluidonly