CI: Add SDL2 and SDL3 build matrix

Document the supported SDL configurations and build dependencies for each
platform. Keep Android on SDL3 only and identify every packaged artifact by
its SDL version.

.github/workflows/linux.yml

@@ -3,33 +3,38 @@ on: [push, pull_request]
 
 jobs:
   build:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-26.04
     strategy:
+      fail-fast: false
       matrix:
         build-type: ["Debug", "Release"]
+        sdl-version: [2, 3]
 
-    name: Linux ${{ matrix.build-type }}
+    name: Linux SDL${{ matrix.sdl-version }} ${{ matrix.build-type }}
     steps:
     - uses: actions/checkout@v6
 
     - name: Install Deps
       run: |
         sudo apt update
-        sudo apt install libsdl2-dev libfreetype-dev libsdl2-mixer-dev libwebp-dev libportmidi-dev libcjson-dev asciidoctor gettext
+        sudo apt install libfreetype-dev libwebp-dev libportmidi-dev libcjson-dev asciidoctor gettext
+        if [ "${{ matrix.sdl-version }}" = 2 ]; then
+          sudo apt install libsdl2-dev libsdl2-mixer-dev
+        else
+          sudo apt install libsdl3-dev libsdl3-mixer-dev
+        fi
 
     - name: Build
       run: |
-        mkdir -p out/${{ matrix.build-type }}
-        cd out/${{ matrix.build-type }}
-        cmake -DCMAKE_BUILD_TYPE=${{ matrix.build-type }} ../../
-        make -j4
+        cmake -S . -B out/build -DCMAKE_BUILD_TYPE=${{ matrix.build-type }} \
+          -DXSYSTEM35_SDL_VERSION=${{ matrix.sdl-version }}
+        cmake --build out/build -j4
 
     - name: Test
-      run: ctest --output-on-failure
-      working-directory: out/${{ matrix.build-type }}
+      run: ctest --test-dir out/build --output-on-failure
 
     - name: Verify xsystem35.pot is up to date
-      if: matrix.build-type == 'Debug'
+      if: matrix.build-type == 'Debug' && matrix.sdl-version == 2
       run: |
         xgettext --default-domain=xsystem35 --directory=. \
           --keyword=_ --keyword=N_ \