Fix installing autostart files in NixOS

sysconfdir can be absolute or relative, trying to ensure
the file is actually installed to the prefix in NixOS.

data/meson.build

@@ -4,10 +4,11 @@ install_data(
  rename: meson.project_name() + '.desktop'
 )
 
+fs = import('fs')
 meson.add_install_script(
  symlink,
- join_paths(get_option('datadir'), 'applications', meson.project_name() + '.desktop'),
- join_paths(get_option('sysconfdir'), 'xdg', 'autostart', meson.project_name() + '.desktop'),
+ join_paths(datadir, 'applications', meson.project_name() + '.desktop'),
+ join_paths(fs.is_absolute(sysconfdir) ? sysconfdir : prefix / sysconfdir, 'xdg', 'autostart', meson.project_name() + '.desktop'),
 )
 
 dbus_dep = dependency('dbus-1')

meson.build

@@ -2,6 +2,7 @@ project('io.elementary.settings-daemon',
  'c', 'vala',
  version: '1.3.0',
  license: 'GPL3',
+ meson_version: '>= 0.54.0',
 )
 
 fwupd_dep = dependency('fwupd')
@@ -46,6 +47,7 @@ config_dep = declare_dependency(
 
 prefix = get_option('prefix')
 datadir = join_paths(prefix, get_option('datadir'))
+sysconfdir = get_option('sysconfdir')
 
 symlink = join_paths(meson.current_source_dir (), 'meson', 'create-symlink.sh')