ci: Support additional destdir in install-meson-project.sh

To build a system extension we need to:

1. Add dependencies to the container (to build it), and also
2. Add dependencies to the extension (to run it, later).

So there's effectively two locations where we need to install
those dependencies.

See https://discourse.gnome.org/t/towards-a-better-way-to-hack-and-test-your-system-components/21075
See https://gitlab.gnome.org/GNOME/gnome-build-meta/-/issues/837

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3843>

.gitlab-ci/install-meson-project.sh

@@ -13,6 +13,7 @@ usage() {
 	  --subdir       Build subdirectory instead of whole project
 	  --prepare      Script to run before build
 	  --libdir       Setup the project with a different libdir
+	  --destdir      Install the project to an additional destdir
 
 	  -h, --help     Display this help
 
@@ -25,6 +26,7 @@ TEMP=$(getopt \
   --longoptions='subdir:' \
   --longoptions='prepare:' \
   --longoptions='libdir:' \
+  --longoptions='destdir:' \
   --longoptions='help' \
   -- "$@")
 
@@ -34,6 +36,7 @@ unset TEMP
 MESON_OPTIONS=()
 SUBDIR=.
 PREPARE=:
+DESTDIR=""
 
 while true; do
   case "$1" in
@@ -57,6 +60,11 @@ while true; do
       shift 2
     ;;
 
+    --destdir)
+      DESTDIR=$2
+      shift 2
+    ;;
+
     -h|--help)
       usage
       exit 0
@@ -85,5 +93,11 @@ git clone --depth 1 "$REPO_URL" -b "$COMMIT" "$CHECKOUT_DIR"
 pushd "$CHECKOUT_DIR/$SUBDIR"
 sh -c "$PREPARE"
 meson setup --prefix=/usr _build "${MESON_OPTIONS[@]}"
+
+# Install it to an additional directory e.g., system extension directory
+if [ -n "${DESTDIR}" ]; then
+    meson install -C _build --destdir=$DESTDIR
+fi
+
 meson install -C _build
 popd