systemd: Add systemd-system-unit-dir override

meson_options.txt

@@ -179,6 +179,9 @@ option('udev',
 option('udevrulesdir',
        type : 'string',
        description : 'Directory for udev rules (defaults to /lib/udev/rules.d)')
+option('systemd-system-unit-dir',
+       type : 'string',
+       description : 'Directory for system systemd units (defaults to /usr/lib/systemd/system)')
 option('systemd-user-unit-dir',
        type : 'string',
        description : 'Directory for user systemd units (defaults to /usr/lib/systemd/user)')

src/daemon/systemd/system/meson.build

 systemd_system_services_dir = systemd.get_variable(pkgconfig: 'systemdsystemunitdir', pkgconfig_define : [ 'rootprefix', prefix])
+if get_option('systemd-system-unit-dir') != ''
+  systemd_system_services_dir = get_option('systemd-system-unit-dir')
+endif
 
 install_data(sources : 'pipewire.socket',
              install_dir : systemd_system_services_dir)