meson: Avoid using relative paths to po directory

Replace the relative path to the po directory with a toplevel definition
of po_dir and use that instead.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/507

src/calendar-server/meson.build

@@ -30,7 +30,7 @@ configure_file(
 i18n.merge_file('evolution-calendar.desktop',
   input: 'evolution-calendar.desktop.in',
   output: 'evolution-calendar.desktop',
-  po_dir: '../../po',
+  po_dir: po_dir,
   install: true,
   install_dir: desktopdir,
   type: 'desktop'