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.
2 jobs for add-systemd-disable-option in 2 minutes and 54 seconds (queued for 6 seconds)
Status Name Job ID Coverage
  Build
passed build-fedora #1645875

00:02:54

failed build-ubuntu #1645876
allowed to fail

00:02:20

 
Name Stage Failure
failed
build-ubuntu Build

|Executing subproject libadwaita method meson
|

subprojects/libadwaita/meson.build:1:0: ERROR: Meson version is 0.56.2 but project requires >= 0.59.0

A full log can be found at /builds/knuxify/xdg-desktop-portal-gnome/_build/meson-logs/meson-log.txt
Cleaning up project directory and file based variables
ERROR: Job failed: exit code 1