build: allow building with system gtk-frdp if found
This way we don't hardcode gtk-frdp to be a submodule for non-flatpak builds, but still have a proper fallback in case it's not installed on the system.
This way we don't hardcode gtk-frdp to be a submodule for non-flatpak builds, but still have a proper fallback in case it's not installed on the system.