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.