meson.build: add option to disable systemd service file build

On systems without systemd the service file isn't necessary. This commit adds automatic systemd detection (that can be overridden with the -Dsystemd_service option), which determines whether the service file is built and installed.

This solves an issue I had while building xdg-desktop-portal-gnome on Alpine, where the build would fail due to the lack of systemd; I solved this temporarily by force-setting the systemd user service dir to a temporary location, but I figured a proper fix was in order.