Build using GCC 9 in CI

Checking that we don't regress this later.
Ubuntu doesn't package GCC 8, and we don't support it either,
so that is a good compromise for now.

.github/scripts/install_deps.sh

@@ -20,10 +20,14 @@ case "${OS%%-*}" in
 				pkgs="$pkgs libz-mingw-w64-dev g++-mingw-w64-x86-64-win32"
 				TOOLSET=
 			;;
-			'' | lcov)
+			g++-9 | lcov)
 				pkgs="$pkgs libpng-dev pkgconf $TOOLSET"
 				TOOLSET=
 			;;
+			'' | g++ | clang++)
+				pkgs="$pkgs libpng-dev pkgconf"
+				TOOLSET=
+			;;
 		esac
 		sudo apt-get update -qq
 		# shellcheck disable=SC2086 # (This word splitting is intentional.)
@@ -32,10 +36,13 @@ case "${OS%%-*}" in
 	macos)
 		pkgs=bison
 		case $TOOLSET in
-			'' | lld)
+			lld)
 				pkgs="$pkgs $TOOLSET"
 				TOOLSET=
 			;;
+			clang++)
+				TOOLSET=
+			;;
 		esac
 		# macOS bundles GNU Make 3.81, which doesn't support synced output.
 		# We leave it as the default in `PATH`, to test that our Makefile works with it.

.github/workflows/testing.yml

@@ -40,6 +40,9 @@ jobs:
         exclude: # Don't use `g++` on macOS; it's just an alias to `clang++`.
           - { os: macos-15-intel, cxx: g++ }
           - { os: macos-26, cxx: g++ }
+        include: # Use `g++-9`, the earliest GCC version we support, on the earliest Ubuntu runner.
+          - { os: ubuntu-22.04, cxx: g++-9, buildsys: make }
+          - { os: ubuntu-22.04, cxx: g++-9, buildsys: cmake }
       fail-fast: false
     runs-on: ${{ matrix.os }}
     steps:
@@ -47,7 +50,7 @@ jobs:
         uses: actions/checkout@v6
       - name: Install deps
         run: |
-          .github/scripts/install_deps.sh ${{ matrix.os }}
+          .github/scripts/install_deps.sh ${{ matrix.os }} ${{ matrix.cxx }}
       - name: Build & install using Make
         if: matrix.buildsys == 'make'
         run: |