ci: Support setting libdir in install-meson-project.sh

Besides building custom fedora images,  we will build
system extensions for GNOME OS. Since GNOME OS uses a
a different libdir compared to what meson defaults to,
we must allow setting libdir in this script.

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

@@ -12,6 +12,7 @@ usage() {
 	  -Dkey=val      Option to pass on to meson
 	  --subdir       Build subdirectory instead of whole project
 	  --prepare      Script to run before build
+	  --libdir       Setup the project with a different libdir
 
 	  -h, --help     Display this help
 
@@ -23,6 +24,7 @@ TEMP=$(getopt \
   --options='D:h' \
   --longoptions='subdir:' \
   --longoptions='prepare:' \
+  --longoptions='libdir:' \
   --longoptions='help' \
   -- "$@")
 
@@ -50,6 +52,11 @@ while true; do
       shift 2
     ;;
 
+    --libdir)
+      MESON_OPTIONS+=( --libdir=$2 )
+      shift 2
+    ;;
+
     -h|--help)
       usage
       exit 0