check systemd unit dir from systemd.pc in meson

NEWS

@@ -7,6 +7,7 @@ ver 0.23.13 (not yet released)
   - pipewire: fix corruption bug due to missing lock
 * Linux
   - shut down if parent process dies in --no-daemon mode
+  - determine systemd unit directories via pkg-config
 
 ver 0.23.12 (2023/01/17)
 * input

systemd/system/meson.build

 systemd_system_unit_dir = get_option('systemd_system_unit_dir')
 if systemd_system_unit_dir == ''
+  systemd = dependency('systemd', required: false)
+  if systemd.found()
+      systemd_system_unit_dir = systemd.get_pkgconfig_variable('systemdsystemunitdir')
+  endif
+endif
+if systemd_system_unit_dir == ''
   systemd_system_unit_dir = join_paths(get_option('prefix'), 'lib', 'systemd', 'system')
 endif
 

systemd/user/meson.build

 systemd_user_unit_dir = get_option('systemd_user_unit_dir')
 if systemd_user_unit_dir == ''
+  systemd = dependency('systemd', required: false)
+  if systemd.found()
+    systemd_user_unit_dir = systemd.get_pkgconfig_variable('systemduserunitdir')
+  endif
+endif
+if systemd_user_unit_dir == ''
   systemd_user_unit_dir = join_paths(get_option('prefix'), 'lib', 'systemd', 'user')
 endif